I agree wholeheartedly with the advice people have been giving to pick what you personally like using-possibly with some bias towards either Canon or Nikon due to their fuller systems than Pentax, Minolta, Olympus. There are a couple of additional considerations, based on the particular type of photography you do.
If you think you may want a REALLY high resolution, cost no object camera in the future, Canon has been better about introducing those than Nikon-they cost $8000, but Nikon hasn't been competing with them. On the other hand, Nikon has the D70, which is the best $1000 body out there right now (I wouldn't consider any Canon Rebel, because the controls are too hard to use-if you go Canon, get at least a 20D or even a used 10D). Nikon also has a VERY interesting $2000 body (if you can still get one). The D2h doesn't get a lot of mention around here, because it's "only" 4.1 mp. Don't let that fool you-I own one, plus a D70, and the D2h gives better prints.
What's special about the D2h is that it is an ultra-rugged professional camera with fully professional metering and focus for $2000. Every other digital SLR under $4000 has the heritage of a $300 film camera in it-the D2h has the heritage of a $2000 film camera. If you want or need the build quality, the incredible AF or the very high (8 fps) frame rate, and you don't have $4000 for a Canon 1D mk II, the D2h is your only choice. If these features don't matter to you, it is heavier than you need, not to mention more expensive. As for the 4.1 mp, I print 12x18 inches from mine all the time, and am very satisfied with the quality. I have printed 16x24, and I'm still satisfied (although I know that is close to the limit).
Either Canon or Nikon are great choices, and each has a special camera or two that the other one can't compete with. I don't know Canon well, but there are plenty of people here who do.