Interesting info. I feel I can't trust the metering in my 60D in Av in lower light situations. I'm shooting in manual 90% of the time now. Even with a flash connected, the shutter speeds my camera picks in Av are waaaaaaay to slow and i always end up with a missed/blurry shot.
Rember in Av mode with flash, the camera is metering the shutter speed for a proper exposure of the background. So essentially, it assumes the flash will not have an effect on the background, only the closest objects. Av mode is not good for hand held flash stuff. IMO. I always use manual with flash, whether I'm using the flash in ETTL or manual mode.