Careful of some international cases. If they are a super snug fit, they won't fit well with the US AT&T version as it is slightly thicker (and i do mean very slightly).
I bought a mesh case off ebay for 2 bucks that was for the international GS2 and it doesn't fit right. The sides don't quite wrap around the edges and it constantly wants to pop off. Kind of sad, but at least i didn't drop $30+ for a case this time.
I'm thinking of buying a TPU/Gel case for a buck in the mean time - those will probably fit ok since they arn't a hard mold. If you do want a hard plastic case, I would make SURE you are buying one with it pictured on an AT&T GS2 (just make sure you see the at&t logo and no home button on the phone).