Even though I have a Canon Rebel Xt, I'd agree that the Nikon D40 and D70 are also great choices, as is the Sony Alpha 100. Those are 3 "beginner" DSLRs. With any, as stated above, you'll eventually want to invest in a good Macro, in the 50 and 100mm range. Keep in mind that once you make the jump to DSLR, it gets very expensive. Most lenses will cost you more than the camera...lol. With my Canon, I use the kit 18-55mm, a Tonika 70-300mm Macro and the 50mm 1.8. These are all inexpensive lenses and take decent pics. I'd really love to get the 50mm 1.4 Macro and a 100mm Macro. These are MUCH more expensive.
As I mentioned earlier, there's a HUGE learning curve when switching from P&S to SLR. It usually becomes a second hobby, one that rivals reef keeping when it comes to enjoyment, addictiveness and expense.