Got it this morning. Kernel is 3.4.0-812098 and build is JDQ39.I337UCUAMF3
However there is a method to protect root on rooted phones before updating to MF3 and then re-install root. This is quite simple in fact. It works because the MF3 update only upgrades individual programs and doesn't flash a complete new image as 4.3 does.
This method can be found at Successful Update to MF3, and Retaining Root - xda-developers
I just did the Update, and the major upgrade I see is that the "On SD card" function has been enabled for AT&T in the Application Manager, so you can move all moveable apps to the SD Card now...you may have been able to do this if you rooted your S4, but I never did, so I just moved all the moveable apps over today.
Would love to hear any other new features people see as well, haven't done much other poking around with it yet...just very happy to finally have the "On SD card" function.