When I shot film for a million years, I shot Canon and Nikon and was always very happy with the qualithy. When I moved up to a DSLR, I started with a Nikon D100, and I've shot a lot of commercial work with it and like another poster has commented...never heard any complaints about quality. I've just recently moved up to the D300 and the quality is superb. I find it hard to imagine any situation where I feel it couldn't do the job.
Others make a good point about lenses and accessories. I think a high quality flash (one that's programmable), a GOOD tripod, and a real bag (like a Domke f2) are a must and should be considered. If you spend all of your money on the camera alone, you really limit yourself.
If I had to make a choice (which I have with the D300) I'll spend less on the camera body and more on lenses and accessories that help me expand my capabilities as a photographer.
Steve