Was it my imagination, or did Matt Smith throw open the door to the possibility that he's not back in 2012? On the one hand, he brought up that Tom Baker played the Doctor for seven years, but his whole body language changed when Craig asked him about his own future on the program.
People who tend to know what they're talking about have suggested that the real reason for splitting next year into two series is to allow him to fulfill his three series contract in two years.