An upcoming episode in the Doctor Who spin-off show, The Sarah Jane Adventures, will confirm what Who fans (a.k.a. Whovians) have long suspected: The Doctor himself can regenerate as many times as he likes and is therefore, in essence, immortal.
The 1976 Doctor Who episode, "The Deadly Assassin", revealed that the Time Lords - the alien race to which The Doctor belongs - are limited to regenerating twelve times, but Whovians have long suspected that this rule would change, especially now that the long-running sci-fi series is more popular than ever.
This revelation does not completely violate the already-established rules of the Who universe, as it were. It was never clarified whether Time Lords were incapable of regenerating more than twelve times OR if they merely were not supposed to - seeing as that The Doctor is the last of his kind and that he need not obey any laws of Time Lord society if he so wishes, it seems current Who showrunner Steven Moffat and his fellow writers have decided (*knock on wood*) that the latter was the case.
A lot of Whovians would also point to The Master - an insane Time Lord and arch-nemesis of The Doctor - as having already disproved the "twelve regenerations total" theory. While The Master has only been played by six different actors on Doctor Who to date, the show has long implied that the character has regenerated more than a dozen times already - either way, Doctor Who is set to remain on the air for an even longer time now.
Matt Smith currently plays the eleventh regeneration of The Doctor and there's been no word yet on how many more seasons he will be on the show. Now that we know the Doctor after Smith's won't be the last, there's a better chance that Whovians will get to see a version of the character who is neither male or caucasian. Perhaps The Doctor's wish to be a ginger will at last be realized?
A lot of longtime Who fans will be less than receptive to this revelation about The Doctor, but they ought to remember - there could always be a future twist in the plot that affects the Time Lord's ability to regenerate or something along those lines. We'll just have to wait and see what exactly lies in the future for Doctor Who hereon out.
Source: BBC (via The Guardian)