Didn't the BBC's announcement of the special being in 3D also confirm it was an hour?
I don't think so (and as you might have noticed I'm pretty obsessed with nailing all this stuff down). However, in purely practical terms they've only got a month or so to shoot in before Matt heads off to America, which limits how long it can be. (And shooting in native 3D slows things down too.)
The BBC Drama Controller Ben Stephenson did suggest that the Anniversary/Xmas Specials could form a two-part story, though I imagine that would result in some annoyed people who've paid to see the former in the cinema only to be greated with a TBC at the end.