Android 4.1 is being sent via OTA to European users as we speak. Hopfully this mean U.S based users on Verizon Wireless, T-Mobile and AT&T will be seeing it very soon.
Jelly Bean has taken lag out of Android. Expect better widgets, offline voice typing, expanded notifications, Google Now, two home screen modes, and a overall better user interface. It's important to remember that the Galaxy S III will use Samsung's TouchWiz interface, so it's not yet known if all of Android 4.1 (Jelly Bean) features are available to use.
Android powered Samsung Phones that will receive the Jelly Bean update in the near future, including the Galaxy S II, Galaxy Note, Galaxy Note II, Galaxy Tab 2 7.0, Galaxy Tab 2 10.1, and Galaxy Note 10.