From an article on Canon's Learning Center about the 5DM3 and video:
"* HD 50 fps or 60 fps recording places the greatest demands on memory card speed, especially during ALL-I recording. Required read/write speed during HD 720p ALL-I recording for CF cards is 30 MB/sec; for SD cards, 14MB/sec."
Anyone know why it wouldn't require the same MB/sec rating for both types of media? the MB/sec should be driven by the video recording parameters; not the card format shouldn't it?