The Daily Mirror had an interesting article about this. Although I don't trust it as a source, the regeneration issue is actually more complex than that. Since the inclusion of the War Doctor, the current Doctor is actually on his 12th body. However, the Tenth Doctor apparently used up a regeneration in the episode "The Stolen Earth", after he was shot by a Dalek. Being attached to his appearance for some reason, he healed himself and siphoned off energy to...well...a previously severed hand, which grew into a half human, half Time Lord clone of himself. If this is the case, then the current Doctor has used up all of his regenerations and is on his 13th and last body, which means...they need to be clever, since we already know there's going to be another Doctor after Smith leaves at Christmas.