The debate rages on...
For consideration, an underwater housing with 2 ports, DLSR and 2-4 lenses, 2 strobes and all the batteries, chargers, filters, etc will cost somewhere around $4K for the most entry level camera. a more mid level DSLR setup is closer to $10K.
In effect, the cost of the camera body is a small (somewhere around 10%) of the total cost of your setup, but once you start, you want to buy a camera body that will last a while. You can't just upgrade when you outgrow it because in addition to the $500-$2000 you put into your camera body, you have $1000-$5000 in a housing designed specifically for that camera body.
For this reason, I think it is smart to not get an entry level DSLR like the D40 or the Rebel XTi. Both are not well set up for manual shooting (1 command dial). The Nikon D80 is supposed to be a very good camera, but not nearly as rugged as the D200, which I own and can testify to it's superior build. Otherwise, the D200 has a few more options, but the 2 are comparable. I understand that the Canon 20D is a great camera, which many like. The 30D is the successor, but was not significantly different from the 20D for most to upgrade. Many are waiting the the next Canon in that range.
Regarding lenses, the big difference between Canon and Nikon was that Canon didn't have a 180 deg fisheye lens for a cropped sensor camera like Nikon did (10.5mm) that changed when Tokina came out with the 10-17mm FE zoom lens for both canon and Nikon. It's a wonderful lens and should be considered as one of your first UW lenses (But is pretty useless above water, so your budget might call for duel use lenses)
Regarding the "other" makers of really good SLR (Sony, Olympus, Fugi, etc) be careful,they make great cameras, but there are fewer housing on the market, so your selection of housings will be limited.
Just a few thoughts off the top of my head.
John