you know, all you have to do is pull the flashes out of the room that your shooting in. i have the 7d and i did the same thing. i turned out the lights, fired a couple of shots with my 580 and 430 EXIIs and then pulled them out to be certain no light was coming from the on board flash. i took the shoot and the image was completely black. i was about three feet away from my subject so if there was an effect from the on board flash it would have showed up for sure. we can rule out this being a problem.