Google revealed the Android 4.1 Jelly Bean in the Google I/O 2012 sometime ago. However, Samsung has confirmed that the Samsung Galaxy S III will get "updated to Android 4.1 Jelly Bean with TouchWiz UI", and according to some reports the International version and the U.S. version will get the "update via KIES during Q4,2012."
Reports also suggest that Samsung has upped the RAM of the Galaxy S III to make the phone "future-proof" as engineers weren't 100% sure whether 1GB of RAM would be enough for Jelly Bean. Hence they opted for 2GB RAM.