Yeah if your a canon guy and not wanting to pay for the prosumer models yet (40D, 50D, 5Dmk2) then XSi is awesome. Its basically a 40D camera minus the magnesium body and a few pro features. A great starter SLR camera.
I don't know much about Nikons, but basically those two brands are the only ones I'd consider since they have the largest following and thus the largest amount of interchangeable lenses and accessories to utilized as your interest grows.