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Actor Matt Smith Will Be 11th Doctor Who
Jerry Smith was among a large number of readers letting us know that the 11th Doctor Who has been named. It's Matt Smith, 26, who will be the youngest actor to play the time-traveling Doctor. The head of drama at BBC Wales said this about Smith's audition: "It was abundantly clear that he had that 'Doctor-ness' about him. You are either the Doctor or you are not."
http://en.wikipedia.org/wiki/Regeneration_(Doctor_Who)#In_the_series
The BBC's Series 4 FAQ suggests that now the Time Lord social order has been destroyed, the Doctor may be able to circumvent the limit on regenerations; it says: "Now that his people are gone, who knows? Time Lords used to have 13 lives.
In "The Sound of Drums" (2007) the Master is revealed to have been granted a new body by the Time Lords during the Time War with at least one new regeneration. Non-Gallifreyans are also seen to regenerate in Underworld (1978) and Mawdryn Undead (1983), but with adverse side effects.
Sounds to me like regeneration is a socially-imposed limit to keep them from living forever. ;)
But they aren't immortal... found this interesting tidbit:
In The Mind of Evil the Master points a conventional firearm at the Doctor and threatens to "put a bullet through both your hearts", while in "Forest of the Dead", Professor Song warns that an impending electrocution would stop both the Time Lord's hearts, killing him. From these, it is apparent that a Time Lord can die if both his hearts stop.
This quote also further supports it:
The TARDIS appears to assist in the regenerative process. In addition to the second Doctor's explicit statement to this effect shortly after regenerating from the first, regenerating outside the TARDIS has never been shown to go particularly well. Of the four occasions on which this has happened, one is forced on him by the Time Lords (The War Games), one requires a Time Lord to give the Doctor's cells a "little push" to start the process (Planet of the Spiders), one needs the TARDIS's "Zero Room", a chamber sealed from all outside forces, to help him recover (Castrovalva) and the last occurs a few hours after he has actually "died" (The 1996 television movie). That last regeneration remains the only one that takes place significantly far away from the TARDIS, without any obvious interaction from other Time Lords, though it may be noted that in The Doctor's Daughter, Jenny - a woman created from the Tenth Doctor's DNA - dies and later reanimates in a process that has some apparent similarities to a regeneration, some time after the TARDIS leaves her planet.
All these + more indicate that the limit may not be a physical one.
Another:
In "Last of the Time Lords", the Master and the Doctor demonstrate that regeneration is not an automatic process (or the process is automatic but the Time Lord undergoing it can halt the regeneration at will) as, despite the Doctor's pleas for him to regenerate, the Master instead chooses to die after being shot by Lucy Saxon
It's quite possible that it's a socially imposed limit - that is, multiple timelords can collectively decide whether you get to regenerate or not.