I was testing my 580EX that I just got back from Canon today and it seemed weird. I sent it in because it would only fire at full power in Manual flash mode (Av on camera). Don't ask "how do you know... etc." It did, I know. They fixed it and replaced some other things on it and now it works. I did have a question though about how manual flash works on the camera because it was acting weird (my other 580EX was doing the same thing, so I'm guessing it's correct, I just don't understand WHY it was doing it...).
30D, 50mm 1.8, 580EX on camera
Metered the dining room without flash in Av mode... 1/25, f/1.8, ISO 100
Turn the flash on, set to Manual flash, 1/128th power... and it changed the shutter speed to 1/250th (x-sync). It didn't matter what the ISO was or what the f-stop was... always 1/250th.
Then I tried setting it to Program and still Manual flash, 1/128th power. Now pointing it at the same thing in the dining room it was saying 1/250th, f/22 blinking. Definitely not right.
Like I said before, both of my 580EXs were doing this so it must be normal, or something wrong with the camera (I'll try my other 30D tomorrow). What do you think?