With any D-SLR, there's a modest concern of which system you're slowly but surely locking yourself into. If you buy Nikon or Canon, you've got an impressive after-market of used equipment. If you buy anything else, things get a little rougher.

That said, Nikon's bottom-of-the-line D40 cuts one very specifically important corner. All but the newest Nikon lenses expect the motor that drives the autofocus to be inside the camera. The latest Nikons have the motor in the lens, so the camera only needs to supply power. Part of how they cost-reduced the D40 was to remove the in-camera autofocus motor. This may not actually be all that big a deal, since you can learn your way around the D40 then upgrade later to a fancier body, should you be so inclined.

Alternately, you can consider the used market, where $500 should be enough to get a Nikon D50 or D70 plus a reasonable starter lens.