Is there a release date for it yet, and if I update to it when it comes, can I easily switch back to 4.1.1 if I don't like it?
I'm new to the android world so there are quite a few things that I'm still learning.
They don't give dates for these updates. If they decide to push it out, they'll either say it's coming soon or they won't say anything until it's out and then make an announcement. The carriers got away from giving dates because if they run into a snag and have to delay it, all the impatient jackasses who don't understand what goes into these kinds of updates start screaming and yelling, saying that Samsung & AT&T lied, and blah, blah, blah. So they don't give dates any more.
If you update to 4.1.2, you won't be able to revert to 4.1.1 unless you root and either (1) do a nandroid on your phone before updating; or (2) download and flash a stock 4.1.1 image from AC or XDA forums.
There is a fairly decent chance, though, that Samsung may skip the 4.1.2 update for the GS3. I think a lot of the new functionality, other than photosphere, has already been added by Samsung, so they may skip it and provide us with 4.2 later on. No real rumors either way yet.