Sorry to be spamming my XSI findings, but all of these questions have been asked here recently and I haven't seen them answered.

I never used the 40D's auto ISO. I usually shoot in Aperture priority, and the 40D would just go between 400 and 800 ISO unless the max shutter was hit and then back down to 200 or 100. Not at all what I would want it to do.

The XSI defaults to 200 for the most part and will go to 100 if max shutter is hit, or will go to 400 or 800 of the shutter speed is too slow (about 1/focal length). So the shutter speed remains about constant as it goes through ISO200-800.

Aside from not going down to ISO1600, this is exactly how I would want it to work!!! Why the 40D defaulted to ISO400 I'll never know.

Again...another nice surprise! I'll start using AutoISO I think, since it defaults to doing what I would want it to do for the most part.

Rick