av and flash

How come in Av mode, my camera doesn't recognize that there is a flash? It will meter for extremely long exposures.

Is there something I'm missing?

Your camera does recognize that there is a flash (there should be a lighting symbol in the viewfinder to indicate that the flash is on). However, what the camera doesn't do is set the shutter speed for you like some other brands might do. If you want the camera to set the shutter speed for you when using the flash, then you should shoot in the P mode. If you want full control over your shutter speed and your aperture, then M mode is the mode to use.

Av and Tv modes are for automatic metering of ambient light. If you're not relying on ambient light for your exposure, there's no reason to use them.

I was wondering the same thing. Thanks for the information.

AV mode with a flash will basically give you fill-flash good for highlighting your subject in shadow or against a brighter background.

For fully automatic flash photos, use Manual mode on your camera, set the shutter speed and aperture to taste ignoring the light meter and let the flash take care of the exposure automatically.

Custom function 3 on a 350D. setting it to 1 fixes the shutter speed to 200 in AV.