I agree with the above posters that the 40D is the best choice at the moment. I bought one about 4 months ago, and I shoot with it every day. I do not regret buying the 50D one bit. The few minor updates do not really justify the cost differential. Also, 10MP is more than adequate for the stuff that I shoot. If 15MP is really that important to someone, then that person should be looking at a full-frame DSLR.
Also, I used the savings difference to buy a 50mm lens and am looking to get a 28mm lens as well.
Edit, I also wanted to mention that I have had a rebel XTI since September 2006. I would not recommend getting a Rebel or one of the lower end Nikon cameras. You will quickly find that they are too limited and will be wishing that you had upgraded to a higher-end camera.
Also, I used the savings difference to buy a 50mm lens and am looking to get a 28mm lens as well.
Edit, I also wanted to mention that I have had a rebel XTI since September 2006. I would not recommend getting a Rebel or one of the lower end Nikon cameras. You will quickly find that they are too limited and will be wishing that you had upgraded to a higher-end camera.