Any other ATT Nokia Lumia 900s get their updates yet? Maybe try restarting your phone? The odd thing is that when my friend checked her ATT 900 phone for updates, it showed none, but she's never installed a single update, so it should have definitely showed updates. You might try syncing your phone to Zune and see if it finds anything? I
I don't understand why I got the 7.8 update for my 900--I certainly wasn't notified of being a tester or anything, so I don't know!