It depends on how "real" you're looking to go. In my humble opinion, the rumored Mark II version of the Canon EOS 5D will be the sweet spot. It may well be announced at PMA this year. The existing EOS 5D is also very nice. The 24-105mm L-Series lens is a good starting lens to match up with it.
In a more affordable price range, the EOS 40D is very nice.
The best of the current SLR offerings is the EOS 1Ds Mark III, at about $8,000. Plus lenses. (That's not counting medium format SLRs. They're expensive.)
Nikon has been out of the running in digital SLRs for several years, but they recently got back in the game with their latest two introductions. Their new full-frame camera has astoundingly-good low-light performance, but considerably less resolution than the Canon offerings.
