In low light conditions, my first shot will often be handheld with no flash. If there's not enough light however, the shutter speed can be too slow and I'm not steady enough for a crisp image - so I'll pop up the flash.
However, I've found that the camera often decreases the shutter speed (i.e. a longer exposure) even though the flash is firing! I mainly find it when shooting in Aperture Priority mode (with ISO on auto).
Why is this? Surely the additional light from the flash would mean a shorter exposure would suffice?