How do you use flashes to achieve an even, well-lit background when photographing a subject against a green or blue chromakey background? One flash, two? What position? What about light modifiers, like an umbrella or soft box?
I'm no expert on this, but my basic understanding is that you generally want two lights, on either side of the background (not at 90 degrees to it), aimed towards the center, far enough back to illuminate evenly, and angled downwards to reduce spill. Continuous lighting is probably better for this, or at least makes it easier to ensure that you're correctly lit.
In terms of lighting modifiers, it will depend. If you can control the strengths of the strobes sufficiently, you might not require them. That'll probably require some experimentation. In generally, your background light should be about a stop less than your foreground.
Anyways, that's a nutshell summary of various articles I've read and hardly an exhaustive tutorial on the subject. For further info, unless we have a real lighting wizard pop in here to fill in better details, you might want to look at what videographers do, it's the same principle just at more frames per second.
Also, as a final tip, make sure your subject isn't wearing something matching the key. Unless, of course, you're looking to partially remove them. :)
John Cavan's answer is definitely more thorough, but I'll answer from a tiny bit of experience. I took my kids recently to a Discovery Science museum here in Souther California, and they had a cheap green screen setup with a video camera aimed at it. The point was to let the kids try on different costumes and dance in front of the camera and see themselves then on the lcd monitor in the "Under Sea" world. The lighting in there was abysmal, yet things worked just fine on the video screen.
All of that to say that your best bet is to do some testing and figure out how much light you'll need.
Lighting the background for the same exposure as the foreground is the most important factor. If you have a green screen and you underexpose the background, you will see the spill very clearly, and it's tough to post process!
I have only done this twice - so I am no expert either. But both times I have done it, the lighting on the background itself did not really matter - since they will be replaced anyway.
John Cavan's answer is excellent in getting equal lighting on your background - but what I am trying to say is that that effort in doing that might be unnecessary for green screen/chromakey.
Chroma key backgrounds are mostly used in video.
In photography, often a white background is better - it can just as easily be cut in Photoshop, or not at all - since most backgrounds are white (computer screens, plain old paper).
In video, you often do not need to light it evenly, just have some amount of light fall on it. Sometimes the spill from the main light is enough.
If you define the problem as 'How do I evenly light a background' the solution is still simple: You can achieve it with a single light, given that you have enough space. In practice you usually use 2 or 4 lights with no modifiers places as far as possible.