I’m experiencing the same problem with a SanDisk 16GB 32MB/s card, and it drives me crazy! I’ve lost precious video material several times. It happens unpredictedly.
Would there be any way to force Canon to help its customers? To me this sounds like a ‘known issue’. Unfortunately the warranty period of my 7D is over, and this Card Error happens so randomly, it probably would be hard to prove something is wrong with this camera.. I doubt that this is a repairable issue.