I e-mailed Canon support this morning to find out if my Keystone speedlite that I purchased with the film Rebel was compatible with the XTi. I felt I had an issue, dark but flashed pictures. Not cutoff as if they weren't flashing at 1/60 but just not much light. Canon responded this afternoon that my speedlite is not compatible with the digital cameras. Bummer! I really liked it because I could bounce off ceilings or walls, I could set it on wide, normal or one of two zooms for different concentrations of lighting needs. For now, I have to use the pop up, a convenience flash, and impervise if I want to tweak the flash down. I have put scotch tape or wax paper over the flash area in the past. Not very scientific but it works. You can even put colored tissue over the source as long as it is strong enough to push through the weight of the paper.
By the way, this reponse time was less than four hours. The one before this was somewhere between 8 and 10, hard to tell since I sent it on the weekend and at night.