I would go with a Canon Rebel. Partially I like Canon because I like their macro equipment better -- they have the best macro flash on the market and a one-of-a-kind closeup macro lens. You might not need that, though. I do really like their 10-22m EF-S wide angle and 17-85mm IS lenses, too. I'm not sure if Nikon has anything comparable or not, but the 10-22mm is especially cool because you need that extremely low focal length to get a good wide angle shot with the crop factor on these reduced-frame DSLRs. I haven't been following Nikon's lens selection in the last few years so maybe they've caught up, but I do know Canon's is really good. I also know Canon has been doing a good job keeping pace with the most useful new technology, like anti-sensor-dust systems.
I can't offer much more detail than that on the distinction between Canon and Nikon, because once you pick a system there's not much reason to keep up to date on the other one. But I DO very strongly recommend going with one of those two, because they're the most likely to be around indefinitely and they have the best selections of accessories. I also strongly agree with your choice to get a digital SLR rather than a point and shoot, because building a repertoire of lenses and filters will add a lot to your photography and you can upgrade as camera bodies improve without having to buy new accessories.
So go with a Canon or Nikon DSLR. If you want to follow my whim, get a Canon. Otherwise, you can read the endless back-and-forths about the issue online, or just flip a coin. :)
Oh, one more thing. If you do go with a Canon, the Rebel series gives you a great camera for the money. But if you can afford it, take a look at the 40D... don't be turned off by the fact that it's on the "prosumer" level and you're just starting. There are advantages to the slightly higher level cameras which you will enjoy even as a newcomer to DSLRs. The question is just whether those advantages are worth the money, and that depends on how much money you have. Lots of the advantages of the prosumer line are easy to enjoy -- big LCD screen, nicer controls. The difference in picture quality between the Rebel and 40D is so small it's irrelevant, but I prefer to use the bigger camera. (Disclaimer: I don't have the 40D yet, but I wish I did!) I would suggest trying out a Rebel and a 40D at a camera shop and seeing which feels more comfortable for common settings adjustments like aperture, ISO, etc.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist