From my somewhat limited experiences taking pictures out of my friend's Cessna, I can recommend the following:

• The camera body itself will not make a huge difference here. You may consider that most brands create better lenses for their professional bodies (in Nikon, D700, D3) than they do for their prosumer bodies (in Nikon, D300, D5000, D90, D3000, D40, etc), so if you can afford it and you plan to use the camera professionally, you may want a higher-end body. Other than that, the camera is much less importan than the lens itself. Also, brand is a non-issue here.
• Buy the best telephoto zoom lens available for your camera body, with 2.8 flat aperture and vibration reduction or image stabilization. Since you'll be in a fast-moving vehicle, you will need the shortest exposure times possible, and that can only be achieved with professional lenses. Save the slow 70-300 f/3.5-5.6 for ground shoots on sunny days. It will cost a lot of money, but presumably you'll be taking these pictures professionally, so consider it the cost of doing business.
• Do NOT let the lens touch the window pane when you are shooting. That transfers all the vibration of the vehicle directly into your camera.
• Buy a circular polarizing filter. You'll notice that unless you have the clearest of days, there will be smog and haze in your photo. The circular polarizing filter will cut through that and allow you to shoot in more marginal conditions.
• This may sound silly, but make sure the plane you're flying in has a clear window to shoot out of, and ideally a clear window on both sides of the plane. You would think this is a non-issue but I've seen some pretty streaky small-plane windows, and that makes autofocusing all the more difficult and you end up doing a good bit of work post-processing the streaks out afterward.