As for comments like "BrandX is clearly better than BrandY" - ignore them. Canon and Nikon both make outstanding DSLR cameras and associated lenses. Both vendors have their strengths and weaknesses. Nikon has nothing like the Canon MP-E 65mm 1x-5x macro. Canon has no general purpose lens like the Nikon 18-200 VR.
I purchased an XSi and the very highly regarded 100mm Canon macro. I totally enjoyed both so much, I wondered if I should have spent a bit more on the camera and walk around lens.
So for the last week, I have been agonizing over XSi vs 40D vs 300D. Most everything you read will say the Nikon 300d is hands down the best of the bunch (and should be for the $800 or so premium). I used the 300d for 2 hours. While there were some things I liked (hi-res screen, AF system performance, utility of 18-200 VR lens), I just didn't like the feel and button ergonomics. The 300d is cover with buttons, dials and nobs. Some are n-way toggles between icons that are obtuse in meaning.
The XSi has by far the simplest, most consistent, and easiest to learn UI for accessing all camera features. The 40D is a close second, and 300d a distant last. Still, If I was going to be a pro photographer, I might choose the Nikon understanding that the investment in learning all of the controls would eventually lead to me being able to more quickly adjust any camera setting.
As any reef photographer knows, reef lighting is a pain to compensate for (white bal, picture style, custom settings, etc). One feature missing on the XSi that was critical to me is the ability to store different overall camera configs in a few presets. The 40D and 300d have just this feature - when switching to a preset, all camera functions can be changed in one action. For example, the 40d has C1, C2, C3 on the mode dial, each of which can hold a complete config. I have C1 for reef, C2 indoor, C3 outside.
Even though the XSi is cheaper than the 40d, it even beats the 40D in some cases. Today I traded in my XSi for a 40D. Cost an extra $400. Going to the 300d would have been an extra $1230.
As many have said, pick the lenses you will use, and spend your money there - you can always upgrade the body later.