There was an ICS leak for AT&T GS2 (i777) this week (as well as a leak for Skyrocket a day earlier), firmware version UCLC2. It was a supprisingly stable build but still have a few critical bugs in it. From the naming, it is the second build in the month of March. So, assume there will be some testing by AT&T in March and rejected (due to those bugs) and we are looking at another build from Samsung either at the end of March or Arpil. Then, another month or two testing. So, I'd say it will be end of May or June at the earliest for the official AT&T release, assuming GS3 is not coming out around the summer for AT&T. Otherwise, it will be further delayed.