I was able to do it two weeks ago.....but I had to search for the right registration page. The one on the front page wasn't the right one, I found it on another page. It did ask me if I was active, etc., but it let me through.
You need to register for your USAA # first, then try to enroll in the CreditMonitor (which is awesome, daily updates!!)
Starting Score: TransUnion: 532 Equifax: 597 -- 4/15/12