I've read everywhere the Rebel is unable to use the flash only as assist light, because the lack to custom functions to disable the flash firing during the take.
That's not true. If you opens the flash, focus (using it as an assist) and then closes it before the actual firing, the camera recalulates the exposure without flash and you can take the picture with ambient light only. It works in Creative modes, I don't know if it also works in Scene modes (never used them). All of you 10D owner can test if it have the same behavior, because it seems to me more practical to close the flash or not, than navigate thru custom functions every time it's needed to change flash firing.