I've been spending a lot of time chatting with a very kind and knowledgeable indivudal, Doug Kerr, on the subject of how my Digital Rebel calculates exposure when using a flash.  One of the most interesting aspects is that when taking a picture with flash, you are actually getting an exposure that consists of both ambient light and the flash.  This is illustrated this timeline (ignore the absolute times and treat them as relative:)

Consider a subject that is backlit.  If you take the picture without flash, you will end up with a silhouette of your main subject.  If, on the other hand, the camera allowed you to only use flash - and not ambient light from the background - your main subject would be properly exposed, but your background would be underexposed.  By combining the two - flash and ambient light - in a single exposure, you end up with both your main subject and the background properly exposed.

From my discussion with Doug, it goes deeper than this, but this was definitely enough to help me understand how flash interacts with the camera's main metering system.