Um? The AF system gets its light through the lens just like anything else... and the camera AFs with the lens wide open. The reason the AF system is dealing with so much less light is that it only gets the fraction of the light that comes through the mirror; anything reflected to the viewfinder can't be used for AF. So, a wider max aperture does provide more light- both to the viewfinder and the AF system...
bseitz234, that doesn't sound like the system used on current Canon DSLRs as I understand it. To clarify, I'm talking about the phase detection system that uses an array of sensors in the viewfinder area. This system only does its job when the mirror is down and all the light coming through the lens is reflected up into the viewfinder. You may be thinking of cameras that use a partially silvered mirror which splits the light between the viewfinder and the image sensor?
Or perhaps you are thinking about the contrast detection system that operates in live view mode? Let's save that for another discussion.
"The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera."
try this: unmount your lens, and look into the camera. start LV, to flip the mirror up. Look down. See those funny looking slots? That's where the AF system is. Those slots allow the light in, so if that light isn't coming through the lens, and through the mirror when it's down, where is it coming from? You can actually see the semi-transparent area of the mirror, too, if you get the angle right...