David Alan Abramczyk said:
I wonder... How're they gonna explain it when Matt Smith leaves, and the Doctor gets onto his last Regeneration?
There's some speculation on the Wikipedia(The original not he fan wiki though its probably there too) that the 13 regeneration is not so much an inherent biological limit but more of a social one. There were some examples in the government of the Time Lords offered the Master more regeneration though that was when he was in a human body.
And with the Water of Mars episode, The Doctor realizes... he is the last, he answers to no one but himself. But... he knows his death draws near. All of his previous deaths, its been sudden. Too quick for him to have any emotional reaction. But this time he's been given a warning.
He will knock four times.
And now there is a Master-race...