I've been using SanDisk Ultra class 4 cards on T2i and I haven't had any problem so far. I happened to have it so I started using with the new Canon cam not knowing that it is technically below the recommended spec - class 6.

What confuses me is the actual speed of this card. Although it says class 4, it also notes 15MB/s on it and I wonder if it is fast enough for writing video files. For test purpose I just kept it recording as writing this post and it recorded fine until it reached 4GB size limit (13' 41" length) without the buffer gauge popping up.

I still wouldn't recommend buying a class4 card, but I wanted to share my case of successfully using this unusual class 4 card from sandisk on T2i.