From the Samsung S3 JB update page, this update will only be available OTA. (Sorry I can't post links yet.)
How Do I Get This Update?
This update is available through Windows and Mac computers using the Samsung Kies software application only. This update will not be made available over-the-air.
This is the link to the part of the Samsung site, talking about the update that is saying it will not come out OTA. Meaning Samsung is pushing out JB, to quiet all of us that have been clamoring for it. Meanwhile AT&T is still working on it, and when they do the OTA, in "the near future, a few weeks", it will be 4.1.2 most likely, although that is just a guess on my part.