I've been debating a DSLR purchase for some time now...
Between a Rebel XT or 20D, Nikon D70s or D50, and toyed with the idea of an Olympus E-300.
Still haven't decided, and as you know, that field is changing pretty rapidly, so new options are always coming up it seems...
If you have lenses for a SLR, and switch to DSLR, the biggest thing a photographer will notice it magnification factors.
Assume a 28mm lens designed for a SLR.
The magnification factor on DSLR's will multiple that by its sensor's magnification factor, anywhere from 1.5 to 2. So wide-angle lenses get narrower due to a non-full size (non 35mm) sensor.
There are a few DSLR's out there with Full-Frame sensors, but until you hit the lottery, they are probably out of most peoples price range, well, it wouldn't make much sense unless photography was a money making career to own one of these.
One bell you might want is the remote shutter release if it has one. Avoid the shakes when you take a picture while it is mounted on a tripod.
Most DSLR's don't have an actual cable release mount and rely on a remote control.
If you want to see some really nice chat on the whole topic applied to aquariums...,check out: http://aquatic-photography.com/