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Actually, I have been reading into this very subject and here's what I've found out.
1. Though Matt Smith is playing the 11th Doctor, using the rule of 12 regenerations would mean that the next Doctor is the last because the 10th Doctor triggered 2 separate regenerations.
Remember, that's 12 regenerations, 13 forms max. In the Doctor's case, 12 would be it.
2. The current show runners seem to have rejected a lot of things that Russel T Davies has introduced in the past. Among these is that the Doctor's regeneration ability has greatly increased. In series 6, there was constant mention of the "Fall of the 11th". From the connotation, it would seem that the Silents are trying to prevent the raise of the 12th Doctor.
3. There was a character in the old Doctor Who run called the Valeyard. He was a culmination of all of the Doctor's darkness, as well as the Doctor's final form. He even tried on occasion to murder his past self. There are numerous hints that writers are thinking of incorporating the Valeyard into the current run.
4. Russel T Davies often used the Last Great Time War which happened between the two runs of the series as an excuse to change anything about the Doctor Who universe. This also included the regeneration cap. His basic notion was probably that if the Time Lords were going to survive an all out assault from the Daleks, then Rassilon would have needed to change the limit he'd imposed on the regeneration cap. Another possibility is that now that the Time Lords are gone, The TARDIS is what powers his regenerative capabilities, with no hard limit on the amount.
But it's clear that the current writers don't much care for those notions. They imposed a serious limit on River Song's regenerative abilities. She used an unknown amount of her regenerations before trying to kill the Doctor and than used the rest to save his life. If River had a large amount like the Doctor, than it would have been very improbable for her to burn through them all to revive the Doctor.
Needless to say, the writers are responsible for all of the confusion regarding the current state of the Doctor's mortality. Only time will tell what they decide to do.

River gave her remaining regeneration to 11 so most likely there will probably be 4 to 5 more doctors depending on how many she had left to sacrifice (donate) to him.
I would, however, love to see a ginger doctor. Just so he can find some satisfaction in it for however long his regeneration lasts. :3

Why do people seem to think that casting a woman as the doctor is a problem? Maybe it looks silly to humans, but did it occur to anyone that maybe gender just isn't that important to Time Lords? In the past yes we've had doctors that strictly like females. But to be honest that is a product of current society, and may change in the future. I am just saying the world is changing and in 20 years that subject may just NOT bother people at all. Shows generally go the way of society, so I wouldn't be shocked at all frankly... Now I haven't watched the whole show but... like, is there a RULE somewhere in past episodes that states that you can't have two time ladies or two time lords who like each other?

Not that it matters much. Because based on what silversonic said (which I totally agree with and BBC SHOULD clarify!) canon was basically stomped on and thrown out the window a while ago. I guess that's to be expected with a show with so many different writers and actors over the years, but they should try to maintain SOME consistency, and try to cover it up at least if they are changing things, by explaining why in a semi-okay manner in the show, so it's not as WTF to us.

This. Although if we do get a female Doctor, wouldn't the companion be male? Or maybe not.

Hm, I guess we'll have to see. I like 11 a lot, but I'm curious about who's coming next. =3

who's to say she would care?