I'm new to the world of DSLRs, but from my research, you can't really go wrong with Canon or Nikon. They are the two 'big' names in the pro market if you want to get really serious, and th emost popular.
I brought my first SLR, a Sony A200 last November, but that was a bit of an impulse buy as it was at a bargain price. I then did some proper research and decided to go for either a Canon or Nikon mid range body as I just preferred the handling to entry models. As I had only invested in one lens, it was relatively easy to sell on. The fact that the Canon used CF cards (like the Sony) and the 40D was better than the D80 and D200 from the Nikon range, and the D90 was too expensive, it was the best choice for me.
I have my 40D for almost 2 weeks now and it is an awesome camera and I'm very happy with it. Just need to learn to take some good photos now!