Okay everybody... check this out. I was looking for a way to get Visual Voicemail working on my One XL... and ran into a dead end. Finally got in touch with a helpful ATT "Advanced Tech Support" guy (after being forwarded from standard tech support)... and he directed me not to the AT&T Visual Voicemail app that is not compatible with the One X... but he says the AT&T Messages app is the one I need. So I installed it... and not only does it work very well for visual voicemail on the One X... the SMS/MMS feature of the app ACTUALLY THREADS GROUP SMS/MMS !!!! And pretty well too.
I've only had it installed about a half hour... but if it's not too buggy... I think this App not only solves the Visual Voicemail issue... but also potentially the SMS/MMS group threading problem. It won't have tons of features for power users (not very customizable... unfortunately)... but for those of us who NEEDED threaded group texts... we may be all set.
(Update: I was not able to post the link because I'm new to the forum... just search for "AT&T Messages" in the Android store.)