Not all videocams use CCD. Many of the new models use CMOS in order to have better lowlight performance and battery life. I have a middle-end professional videocam Sony EX1R that uses CMOS and has a rolling shutter. I also encounter the same problem and though there is software that is supposed to detect all these flash frames, I have yet to find one that has worked as well as I hoped. It is a fairly well-known issue that's been talked about in many video websites as well as wikipedia.
We will have to wait until global shutter is implemented on CMOS before we can be rid of this issue. In the meantime, probably the best thing you can do is stay further away from the photog with the big flash!
