I managed to get my hands on an XSI (450D)for a few minutes and I must say that the camera is a very impressive little camera. For those who have waited and considered the XSi, the time has come and you should really consider checking this little beast out.
I found the XSi to be a very light camera and a slightly better grip feel compared to the XTi. The first thing that was very noticiable to was the large LCD display...very nice! What I immediately found a huge improvement was the larger viewfinder. On the downside, which slightly bothered me, was the battery door. It is very similar to the 40D where the battery door and the latch is completely plastic (no metal). The XSI uses the new LP-E5 battery and the neck strap is new as well, looks like the 20D-40D strap, but black instead of navy.
As a 40D user, the downside of the XSi(IMO) were: the camera did feel a bit small; being used to the rear wheel and joystick, it was cumbersome (for me) changing camera settings on the fly.
As for IQ, not bad...not bad at all. Sorry for the uncreativeness, but here's a pic and a 100% crop. I took the shots with the EF 50 1.4. At ISO 1600, 1/60 f5.6 and spot metering I was quite impressed. Sorry about the pics being soft, but my primary test was performance at 1600 ISO.