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Oops I made a slight error myself on the 24 hour energy budget. It should be 2.58*10^22 joules not 2.25*10^22 Joules. Watts goes to 7.45*10^18 W.

I realize I've arrived late to this party but the idea of terrforming our neighboring planets has always fascinated me.  Clearly Mars will likely be far easier to terraform than Venus.  But I've always had a soft spot for our little hot headed sister world and feel that she could be a valuable asset to human expansion to space if handled correctly.  Two major problems that need to be addressed are her slow rotation and suffocating thick atmosphere. If those two issues are addressed then the other problems (no magnetic field, hot surface, toxic atmosphere) may be easier to deal with and may solve themselves naturally in time.

Unfortunately the crash bang methods (comet/asteroid collisions, nuclear explosions) usually suggested sound too much like planetary rape.  Also I don't think they'll work. Anyone who has seen an animation of two earth sized objects colliding understands that at a planetary scale a rocky crust behaves more like a fluid rather than a solid.  Kinetic energy is absorbed chaotically and a great deal is dissipated as randomized heat energy. The Boltzmann distribution of nuked or impacted atmosphere molecules means only a tiny fraction of the air will achieve the escape velocity necessary to leave the Venusian gravity field permanently. The rest will just fall back with all the heat and radioactive fallout that may take millenia to dissipate. There has to be a better way.

I did some back of envelope calculations to clarify minimum mass and energy needs for terraforming Venus and came up with some interesting results. I'm sticking to the basic joule-second-kg-meter system and scientific notation because using impressive sounding terms like TerraWatt and PettaJoule does not help in allowing others to compare numbers or participate in the conversation. And I’d really like feedback.

Mass of Venus = 4.87*10^24 kg
Mass of Venusian Atmosphere = 4.7*10^20 kg
Escape Velocity of Venus = 10,360 m/sec
Energy to expel all of the Venusian Atmosphere at Escape Velocity = 2.5*10^28 joules

Current Kinetic Energy of Rotation of Venus = 3*10^25 joules
Current Kinetic Energy of Rotation of Earth = 2*10^29 joules

The NASA site says the moments of inertia of Earth and Venus are nearly equal.

Therefore since rotational energy is one half the moment of inertia times angular velocity squared the energy needed to spin up Venus to one day per Earth day is approximately  2*10^29 joules.

Some have suggested that the air could be expelled from Venus along the equator at escape velocity thus not only reducing the total mass of the atmosphere but also speeding up the planetary rotation like Heron's steam turbine.  Inspection of the above data shows that even if we can expel every atom of air we just can't get to a Venusian 24 hour day.  However since rotational kinetic energy is proportional to the square of the angular velocity we CAN get to a longer day.  I estimate that 2.5*10^28 joules can get us to an airless Venus with a SQRT(2*10^29/ 2.5*10^28) * 24 = 67.8 hour day.  Obviously we want some of the Venusian atmosphere left for final terraforming.  I'm assuming one tenth or 9.2 atmospheres (bar). The energy budget drops to  2.5*10^28 * 0.9 = 2.25*10^28 joules.  Plugging into the rotational energy formula above gives approximately = 72 hours or 3 Earth days/per Venus day.

That's a pretty decent spin and could not only help stabilize temperature through atmospheric convection but may also help activate a magnetic field. If the rotational acceleration is applied gently tidal stresses may delicately crack the crust into sections allowing built up heat to release along the fissures and the beginning of a true continental plate tectonics system. This may allow for volcanic release of fresh water vapor into the atmosphere. For the Planet of Love a slow seduction is far better than a brutal assault. 

Otherwise water can be added by means of comet impacts or a small outer solar system object. I've considered Phoebe, Saturn's outermost major moon (approximately 200 km diameter, est. 50% ice) but there may be others. There may be a way of slowly introducing the water to avoid disrupting the crust.  Creating a 1000 year magma ocean is not good for colonization.

So how to do this megaproject? The construction of a massive tower sticking 100 km out of the atmosphere with a powerful blower nozzle has been suggested but as I've said rock behaves like a fluid at planetary scales. Such a device would likely sink into the ground before completion and if not it would topple over once you turned on the blower nozzle. I'm not even going to discuss the challenges of building such a monster in the hot corrosive atmosphere and deep gravity well of Venus.

An elegant solution may be possible. Build a huge solar powered ion cannon and use it to convert Venus into a homopolar motor!  It's easier to build big machines in space rather than on the Venusian surface. And there are many M-class Venus orbit crossing asteroids that could serve as construction material.  The idea goes like this:

1-Place a gigantic solar powered ion beam cannon so that it is always above one pole of Venus. Alternatively place it at the L1 point.  Since an ion cannon is also an ion thruster it could stay there as long as you had fuel to compensate for gravitational attraction and solar pressure. Just don't get too close.
2-Divert a comet or larger water rich object to make an atmospheric grazing encounter with Venus. The fragmentation would make a spinning ring of ionized dust and gas going all the way down to the ionosphere.  The angle of the strike will determine the angle of the ring.
3-Turn on the ion cannon aiming at the perimeter of the ring.  A compensating electron beam will fire where we want the pole to be.
4-As electric charge builds up in the spinning ring a magnetic field will form per the right hand rule.  Electricity flowing outward will accelerate the particles in the ring all the way down to the ionosphere of Venus at right angles to the electric flow and induced magnetic field. In theory the electric flux will reinforce the magnetic field and the ring particle orbital kinetic energies will increase.
5-The particles in the ring perimeter will ultimately reach escape velocity and be dissipated to be replaced by more of the atmosphere.  Ultimately more of the air would be ionized as it spun around the planet with increasing speed.
6-Superhurricane winds are induced from the spinning ionosphere through angular momentum exchange down to the surface causing a slow increase of the planetary spin at the angle of the ring and opposite the direction of the ring.  The ring will be self replenishing as more of the air and some the surface is sucked up ionized and accelerated outward through the electromagnetic homopolar effect.
7-The ring will significantly expand the surface of the Venusian atmosphere, accelerating cooling. However resistive heating from electric flow will have to be factored in.
8- When enough of the air is gone and Venus is spun up enough turn off the ion cannon. If the original comet or moonlet has enough volatiles rain will begin to fall cooling the surface.  The residual ring if formed at a sufficient angle will add to the cooling effect.  Forming rain clouds during the shortened day and dispersing them at night perhaps by using the ion cannon at lower power will also cool things off.
9-If done right the planetary magnetic field and crustal tectonics may be induced from all this activity. Management of the remaining artificial ring and cloud formation will continue to maintain the environment. Then comes atmosphere modification using microbes. Nevertheless the surface will be rather inhospitable until things settle down.

So where to get the energy to do this and how long will it take? Solar energy falling on a solar cell the diameter of Venus at Venus orbit assuming 100% conversion efficiency will produce 2.58*10^22 joules per day.  2.25*10^28 joules/2.58*10^22 joules per day = 872093 days or 2,389 years. This is clearly too long for most civilizations. However if we increase the size of the solar cell by a factor of 5 diameters then the time is reduced to 2389/5^2 = 95.57 years or about 1 century.

The final Venus should be suitable for bioengineered life and modified humans who can handle high CO2 levels. CO2 greenhouse effect will be an issue but an artificially maintained daylight cloud cover can reduce the problem and evening rain will gradually strip it out of the atmosphere to form carbonates.  Solar shading from the residual ring will also help.  And if not a laser cooling effect suggested by karov perhaps using the ion cannon could keep temperatures down. Also the huge solar power cell could be used as a solar shield besides providing virtually unlimited beamed power to the inhabitants of Venus. A true paradise.

Unresolved questions relate to the creation of a self sustaining magnetic field in the spinning ionized ring of electrified air.  Can it be sustained with enough intensity and directional stability to do the work required?  I don't have enough understanding of plasma electrodynamics and ring particle orbital mechanics to answer that question. Perhaps someone out there has access to the requisite knowledge and a powerful computer.

Also the ions fired from the cannon will have to come from somewhere.  Just about any element can be ionized under the right conditions. What to use? Obviously it has to be accessible in sufficient quantities. Hydrogen, oxygen, carbon, even nickel/iron are possibilities.  I’ll leave that for future speculation.

2.25*10^28 joules is obviously well beyond the capacity of a Type 0 civilization like ours.  But that's only 2.25*10^28/3.8*10^26 = 59.21 seconds or about one minute of total solar output.  So a Type I civilization on the Kardashev scale that is trending to a Type II could do it without much problem.