here are several articles on DPReview, thepress release, a brief hands on, and a "whats changed" from the 400D
Canon EOS 450D / Rebel XSi: what's changed: Digital Photography Review
In Brief:
- 12 megapixel CMOS sensor
- SDHC card instead of CF
- Viewfinder sounds to be almost 100% as oposed to croping like previous models
- Image verification data
- Improved 9-point AF system
- Larger, higher capacity, battery
- ISO in viewfinder - excellent, this irritates me on my 350D
- New battery grip... again