I have a 40D and a back up camera (rebel) and I was hoping to get the 580EX but I am not sure if it is compatible with my camera, is it?

I'm almost positive it's compatible with all the Digital SLR's...I believe they all have the same hot shoe mount. My 580EX II fits and works on my old film Canon A2, so I think it would fit on your cameras! I did a quick search, and it looks like the flashes are being sold in kits with the 40d at some stores, so it should be compatible.

Yes, I use a 580EXII on my 40D.

It's compatible with all canon EOS cameras.

Thanks friends

I can confirm that the 580EX II works with the XTi and XT.

I may be wrong on this Dawn, but you should even be able to use your 40D to change flash settings instead of having to make adjustments on both your camera and flash. Sweet! (well...sweet if I've got my info in order!)