What class of SD card does the T2i require for shooting video?

min. class 6

The quality of the card is also very important. Sandisk is pretty much the standard. Many people seem to have good luck with other brands, but it's something to keep in mind!

I can also definitely recommend the card from Transcend - I own three Transcend Class-6 16GB cards for over a year now and they have never given any problem. But I do backup every card on location on an imagetank, just to be sure.