According to a "Community Manager" on AT&T's official forums, the carrier is not planning on releasing the "Windows Phone 8107" bugfix release, which solves an issue related to keyboard disappearances and also likely adds LTE support. That policy stands in stark contrast to what had seemed like a strong affinity for the Windows Phone platform and also in contrast with other carriers both within the US and outside, who have been rolling the update out. AT&T had previously held back on releasing another bugfix update, 7740, and though previous rumors had suggested that carriers wouldn't be able to block updates, that's definitely not the case.
We're not sure why AT&T is opting against releasing this latest bugfix, but it's not because it has a problem with the 8107 build itself — the Titan II on AT&T runs that very version. It's worth noting that even minor software updates have a non-zero impact on a carrier's infrastructure and (more importantly) customer support. We're sure that Windows Phone users who have been dealing with some of these bugs will find that a thin excuse, of course, and we've reached out to AT&T to get more information on the matter.