10 Reasons Why The New Doctor May Actually Be River Song
1. A Birthday Doctor
As seen above, there is a possibility that the answers to this mystery are hidden within Matt Smith's episode 'A Good Man Goes to War'. The largest piece of evidence for the secrets of Martin's Doctor may be in River's own, future, words. At one-point Rory suggests that there may be "two of them", referring to two Doctors. At this River responds "That's a whole different birthday".
Perhaps it's a matter of coincidence - although we as fans should not believe in such coincidences - but the invasion of the Judoon is on Ruth Clayton's birthday. Even though her identity is masked, it is possible that parts of River's history slips through into Ruth's life.
River's birthday becomes Ruth's birthday. Just as River tells Rory, her past includes a birthday in which there are two Doctor's and we may have just witnessed it without even knowing!
But for now, that is all "spoilers".