Big Brother 11: Who is the 13th Houseguest and What are the Cliques?

CBS has announced 12 of the houseguests for Big Brother 11 and revealed that, as rumored, the theme this season is all about high school cliques.  The contestants will play in one of four groups: the popular kids, the athletes, the brainiacs, and the off-beats.  Based on their short bios, it's pretty easy to figure out where each houseguest will be. However, there will be a 13th houseguest, but that person's identity is a mystery until the premiere on Thursday, July 9 at 8pm.  The fact that the houseguest is a mystery strongly implies that it's someone we already know, otherwise there's no point to the mystery.  Will it be a former Big Brother houseguest?  A celebrity?  The unborn child of Julie Chen? (link)