Well, quite a few mentions of it are made in classic Who, though they make it out to be a very final number, and not by choice. The Doctor's mentor (who eventually turns bad for some reason), claims he is on his last regeneration. But yeah, there is the weirdness of the "skeletor" Master, who is described as being on his last regeneration, and for the reason, looks like a rotting corpse... never really explained why he is different. Then there is the Valyard.
Anyway, if Moffat is counting Smith as Regeneration 13, then he's already broken the rules without us even realizing it. Either Moffat is content to just blow the limit off (which I'm just fine with), or it'll need to be explained in the next 3 days as to why it happened, and what's going on. I'm banking on grateful timelords offering The Doctor a full new cycle.
Anyway, looking forward to the big Matt Smith sendoff. It's been fun!