The EVO 4g LTE has it's own form factor and cases are already available from Seidio, Otterbox, and Diztronic.
The S3 has the same form factor as the rest of the world (already available), so I'd guess that these manufacturers already have them and are going to be ready at launch or very close to it.
I don't want to get off topic but sounds like they're keeping the name the same across the board, similar to Iphone. It would be nice if all accessories were cross carrier compatible too.
I compared the specs for the Canadian versions to the international, they were the same.
US versions should be same as those. Should be good for all cases.
I think the Japanese SGS3 is slightly thicker, the rest are the same.