The choice really depends upon on your desires and your previous experience. Do you want to pursue photography for the love of the art, or are you a gearhead? Are you willing to go through a few different cameras to get to the one that floats your boat, or do you need a single solution? Have you used a digital camera before, or will this be your first one? Do you have a big investment in 35mm film gear that might be usable with the same mfgr's digital gear?
The Canons and the Nikons are very nice, though I would make a different choice than Joe and get a used copy of a more rugged professional body like a D1X for what I would spend on a D70 *and* a 20D.
The other DSLRs have good reputations too. The Nikon has the advantage of a bigger range of legacy lenses that fit the current pro bodies, while the Canon can be adapted for use with all sorts of manual lenses including Leica.