AT&T may now be the most prominent carrier of Windows Phones, but even it hasn't got a spotless record of keeping the platform up-to-date. The 8107 build of Windows Phone 7, which fixes a "disappearing keyboard" bug and other issues, still isn't available as an update for AT&T phones. The most recent AT&T forum posts, though, suggest that the company is at least planning to release some kind of upgrade. A community manager has announced that "AT&T plans for a Windows Phone update that will contain the improvements in the 8107 update and more."
As you've probably noted, there's no timeframe given there, and the manager says she'll be posting again when she gets more information. The "and more" also suggests that it could be bundled with Windows Phone 7.5 Tango in a future release. AT&T has previously told WPCentral that it's committed to a post-8107 update like Tango in the coming months, but at this point it's not clear when we should expect that to come out.