I've always wondered about that myself. An EF-S lens is too close to the sensor to flip up a mirror if it's a simple hinge. However, if there's any sort of linkage in there it's quite possible that the mirror could clear. Imagine that the back/top of the mirror translated forward and the bottom/front of the mirror swung up at the same time. That would prevent smashing the close-in lens. It's an expensive (and probably loud) solution so there may be a more elegant one than that - but there are solutions.