DSLRs and HD video cams compress their video signals to set frame rates that usually conform to PAL or NTSC standards. As long as your SD card's write speed tackles the data transfer rate of your video signal out of the camera, then there will be no problem. If not, it will simply stop recording or, depending on the camera, result in very visible frame drops - your signal will be unusable.
JPEG is a compression format for still images, and RAW also refers to still image work. Most HD video is captured with H.264, AVCHD, an MPEG2 variant etc.. These capture streams can be transcoded to video streams with more color latitude for post-processing, like cineform, lagarith etc..
Luckily, some cheap SD cards have write speeds that are much higher than their classification and more than fulfill the requirement for most cameras. See here
for more some tests with cheap class 2 cards on a Canon 550d/Rebel T2i shooting full HD.eyenorth