A Time Lord regenerates (new body and new persona) at the threshhold of death.
You missed it, at the Eccleston as the Doctor finale (Doctor series 9), he “got a bad case of time” and so was forced to regenerate… So along came the ‘tenth’ Doctor by way of Tennant.
The Doctor should have only a few regenerations left before he’s really dead, but I expect they’ll find a way to prolong the Doctor character for many more years to come… His arch-enemy the Master from older series/s had reached the end of his natural Time Lord regeneration cycle and managed to extend his life (the Doctor did not approve).
Incidentally, [url=http://www.youtube.com/results?search=Doctor+WHo+Curse+of+Fatal+Death&search_type=search_videos&search=Search]HERE[/url] is a fun Doctor Who spoof where you get to see the Doctor regenerate several times.