I was there on 12/6 as well. The letter was in a business sized envelope. I actually did not get very excited, since I figured it was a wait list, based on its thinness. I was very impressed with the school and the positive interactions between the students themselves and between faculty/students.
As far as the possiblity of you attending. . . it would be hard to turn down a sunny San Diego acceptance, if one is forthcoming! You should be hearing something soon. Mail probably takes a little longer to get to CA that it does STL.