Windows Phone 7.8 is still officially supported and as such AT&T is now pushing out an update for the Nokia Lumia 900. The 900 already received the 7.8 update that started in January so as such this is more of a bug fix than anything else.
This update bumps the OS version from 8858 to 8862, and according to Nokia this update addresses issues with Live Tiles, adds some theme colors and some lock screen improvements. It also brings updated Nokia apps apparently. The update is cumulative meaning you'll also get previous updates in case you somehow missed them or they weren't pushed by the carrier.
Overall this seems to be very similar to the update that Microsoft pushed a few months back that fixed the Live Tile issues cause by 7.8. The official support for Windows Phone 7.x ends in September 2014 so users can keep looking forward to updates and bug fixes.