Sammobile posted the below screenshots on March 6th from a leaked 4.2.1 ROM for the Galaxy S3.
As you can see, these showcase a couple of new "Smart" features, the same ones that will be featured on the new Samsung Galaxy S4.
This feature will pause the video you are watching when it detects you have taken your eyes off the screen.
This feature will automatically scroll the page when it detects you've read through to the bottom. The screen shots show this feature will work with email and webpages, but it may not be limited to just those. We'll have to wait for more information regarding that.
Also, GSM Israel posted some screenshots that show the same features, supposedly from the American version of the new Samsung Galaxy S4.
So, when will we see this update? You can safely assume that it won't be before March 14th, when the Galaxy S4 is announced.
If I were to guess, I'd say it would be released for the International version some time shortly after the S4 is released. As far as a U.S. release, who knows? We all know the carriers take their time here, and considering that most still haven't released the last update, it could be a while still.