This weekend I was watching the Spurs and Lakers on ABC via the D* HD LIL feed in Austin. I think I see what you folks are talking about, every now and then there would be a flash and the picture would break up, if I flipped over to the ABC feed I would still see the flash but no breakup.
I think some arenas have a system where the presses still cameras are hooked up to a whole house stobe light system that goes off whenever they take a picture. I couldn't see it too well on the D* feed but when the flash happened I could on some scenes see the crowed lit up also. Also the D* feed was worst on the breakup when the camera was panning at the time of the flash, on a steady shot it wasn't too bad, if it happend at all. If that's what's happening I wonder if the MPEG4 encoders just can't handle strobe lights and a lot of movement at the same time???