Until I hear GSP say he'll do it I don't buy it. He would probably lose and might get clowned on, and that is very bad for his image, which he puts above everything else.
He does seem a little aprehensive when it comes to this superfight. To be honest, I just don't think he wants to focus on anything else right now other than the Condit fight. When he wins and Anderson enters the cage to challenge him, I'm sure with all his adrenaline going, GSP will accept the fight.