We know that the Jelly Bean update for US Galaxy S III’s was coming. Samsung said it’d roll out in the coming months. I guess “the coming months” meant this month for Sprint, as it seems that the update will start rolling out tomorrow.
The build is L710VPALJ7 and will include improvements such as Google Now voice searching, Project Butter smoothness, plus the standard TouchWiz improvements you are now used to. The update rolls out tomorrow, but it might take a few weeks for it to roll out to absolutely everyone. So if you don’t get it immediately, don’t fret and just wait a bit longer. Also, thank you Sprint for getting this out so fast!