Much debate over the last couple of days about the exact precise detail of the Doctor's mortality. Allow me to ruminate...

The first step towards regeneration was, as you'd probably know, the unusual step of replacing William Hartnell with Patrick Troughton at the end of The Tenth Planet 4, in October 1966. However this was not at all defined, with the new Doctor terming the event renewal ("I've been renewed."). This had been on the cards for some time, with John Wiles wanting to recast the part following the character's inadvertent invisibility in The Celestial Toymaker. When it came about under Innes Lloyd's supervision, the production team chose a variant on Hartnell's costume, implying the new Doctor was a younger version of the same man. Lloyd also suggested this could be a regular event.

The whole question of how this occurred was avoided in the following such event. In The War Games Troughton Doc was captured by the time lords, and forced to regenerate, or as it was termed "change your appearance". It was only in Planet of the Spiders that the term regeneration was coined, and indeed given another example in the form of the Doctor's mentor K’Anpo Rinpoche. This also established that the time lords were able to create a projection of their future self prior to the change, as the Doctor did in Logopolis ("the Watcher").

However the writers weren't finished with this concept yet. While dueling Morbius in a mindbending contest, we see eight other incarnations of the Doctor as he loses (actually members of the crew). This has been explained in a number of ways (as Morbius' past incarnations or potential incarnations of the Doctor) but was intended to be eight pre-Hartnell Doc versions of the character. Contradicted at every turn, the next addition to the mythology would become the cannon.

In The Deadly Assassin Robert Holmes establishes a finite number of regenerations: 12. This is done to give both an explanation for the Master's new (and horrific) appearance and a reason for his actions, self preservation becoming his standard motivation in stories following this. Twelve becomes the magic number and is confirmed in Mawdryn Undead (eight lives left) with the detail becoming canonical across the show and all spin off media. Until 2008.

The first cracks started to appear in this with a frequently asked questions page on the BBC site. The rhetorical "How many regenerations does the Doctor have?" is answered thus "Now that his people are gone, who knows? Time Lords used to have 13 lives." Fans had been speculating since at least the 1980s on how the BBC would deal with the end of the regeneration cycle, and this had intensified with the quick run through Doctors eight, nine and ten in the 2000s. With the casting of Matt Smith as Doctor eleven, it seemed to be an imminent problem. Would the BBC let its multi-million pound super brand die with its title character, or somehow change the rule in an epic, over-dramatic and operatic season finale, as the style had become under Russell T Davies?

Neither as the case seems to be. Davies left the position of show runner in 2010, and his replacement Steven Moffat seems disinterested in the issue. In response to the oncoming storm of stupidity over the Doctor's mortality he tweeted (on 10 August 2010) "...he's MAKING IT UP, he has no idea." (we can presume he was referring to the Doctor, not some other writer). On Tuesday 12 October the British Film Institute held a preview of the upcoming Davies scripted Sarah-Jane Adventures episodes Death of the Doctor parts one and two. Fans who paid their nine pounds posted a crucial address to the question of the Doctor's mortality online, which has gone on to be massively misquoted. I append the crucial part of the transcript below:

Clyde: So, can you like, change colour?

The Doctor: Yes, of course.

Clyde: How many times can you change?

The Doctor: 507

Ambiguous. What is clear is that the current management is not interested in ending the show with a further two re-castings, or in making another rod for their back.  What is not at all clear is why fans insist on going to the least informed sources with the poorest track record imaginable to define their reality.

May I add that if you're getting your information from tabloid newspapers, some guy's blog or just the last step in a chain of Chinese whispers, you are a frickin' moron and Who fandom would be better off without you.

I agree with what you have said the BBC have not come out with a post on their website or on the offical doctor who website. It is not mentioned or referenced anywhere about what is happening or they plan to happen.

It appears that this talk of "extended immortality"  started off in the Daily Mail which is a tabloid news paper.

Either way I am not concerned it is sci fi they can do what they want to make it fit, some fans happy some fans not.

I don't care, as long as they keep on making Doctor Who!

The Creature from the Pit
1:04:50 (near end of episode three)

The Doctor: Hello every body!
Romana: Are your alright?
D: Yes I'm fine.
R: I was so worried about you.
D: Oh you shouldn't be worried. Time Lords have 90 lives.
R: How many have you gotten through?
D: About 130.

What about in Trial of a Timelord where the Valeyard is revealed to be the Doctor's dark side 'between his 12th and final regeneration'?

One assumes this to mean his 13th persona to be the Doctor's last.

What I'd love to see when the show gets to the end of his 13th self to see a battle between him and the Valeyard - I think there's still plenty of potential for that character and he should come back in a story sometime.

As I recall, Robert Holmes describes him as being between the 12th and 13th regenerations, which was changed to 12th and last to make it more ambiguous. Don't know what the current thinking is, but the BBC Books writers guide discouraged use because he wasn't interesting.