Lets say for a second that I believe the sources on Blogtor Who (I don't, this is just theoretical) then they may be hinting at a return not because it's already been filmed but because they've signed on to appear in the special some time ago. The problems with the sources is that there are a few holes in these bullet points due to what we already know. If Matt is regenerating in a New Year's Day special then why has he been briefed on series 8? (He could be lying through his teeth, I doubt it though.) And we haven't seen any old Doctors on set during the filming of 7.2, and I highly doubt the press would have missed out on a scoop like that. As much as I would love a feature length spectacular to mark the 50th, this just seems too leaky to be believed. Perhaps I'm too cynical.