Den of Geek and Digital Spy are both reporting it, so I expect it will make it into tomorrow's Morning Spoilers, but a stunt actor tweeted that she would be playing a certain reoccurring Who baddie in Matt Smitt's final episode.
Personally, I have no doubt that she's telling the truth, but before we all go reading too much into a single tweet before filming on the Christmas special has even begun, let me just remind everyone that these particular foes also showed up in The Pandorica Opens and A Good Man Goes to War, mainly to be used as cannon fodder.
Considering that Moffat has promised answers and to all of the big questions involving the 11th Doctor, I would be very surprised (and actually, quite disappointed) if that resolution turned out to be "it was the Cybermen all along."*
* Well, unless it turned out to be "it was Mr. Clever all along." I could just about see the super Cyber Controller with a Time Lord brain imprint going completely nutty, and playing everybody, including his own Cybermen, in an elaborate chess rematch spanning all of time and space.
** Though it would be a bit strange if he wanted to off the Doctor before they had actually met, unless of course his goal was to uncreate himself, but that's a bit too much like the Great Intelligence for my liking.