After three separate calls to Apple Support, I was able to reach a representative this morning that finally answered the infamous 5.6 carrier update mystery that quickly followed after iPhone OS 3.1.2 was released last week.
The Apple rep indicated that the 5.6 carrier file update addresses a specific issue that some iPhone users were experiencing after the 5.5 update enabling MMS was rolled out on Friday, September 25th.
When sending photos or videos in an MMS message, the progress bar would stall at about 90% then result in an error ending with a (!) red exclamation point next to the MMS message. The associated message failed to be properly sent, though standard text messages worked as normal. The 5.6 patch addressed that particular MMS issue.
I asked the Apple rep if he believed the problem was widespread.
"I fielded a fair number calls myself with that MMS issue, although it didn't appear to be a widespread epidemic," the Apple rep said, "In some cases it was an AT&T provisioning issue and in one case it was corrected for the user with a simple reboot."
Those awaiting the fix had to wait about two weeks for the 5.6 patch to be issued. While some like myself believed that the 5.6 update may have been a preemptive patch aimed at enabling VoIP apps like Skype to be used over AT&T's 3G network, Apple confirmed that speculation to be untrue.
At least... that's what I was told. Mystery solved!