(Alternately -- and the point that her character's name is Clara, not Oswin, would seem to support this -- it could just be that they're double-dipping like they did with Martha and Amy.)
The more I think about it the harder that is to believe, even if it is the simplest explanation.
Because they kept it under wraps that she was going to be in this episode. And they're smart enough to know that's going to fuel fan speculation, and, while Moffat's track record isn't as unassailable as it was before he had to do this shit full-time, I still think he's way too good a showman to let that kind of buzz go on for three months and then go "Nah, guys, just fuckin' with you; she's just playing a different character."
Even if Oswin and Clara were originally conceived as different characters and the dual-casting decision came later, I'm willing to bet there were rewrites to accomodate it. And something more substantial than "she was Martha's cousin" or "Romana decided to regenerate herself to look like Princess Astra".
It would be better still, of course, to think that it WASN'T a plan that came later and Moffat always intended for there to be some out to save Oswin after all. It does seem to fit his MO.
Hoping, too, that he's not rehashing River's origin and having the Doctor meet her for the first time when she's about to die and then meet her earlier self in a later episode. Though, regardless of the circumstances of their eventual meeting, it's quite possible he won't recognize her -- he never did get to actually SEE what she'd looked like as a human, after all. (Heard her voice, though -- and given that *I* knew I recognized her voice from somewhere, it would be a bit disappointing for the Doctor to once again be less observant than I am.)
MEANWHILE:
Gaiman accepts Hugo for The Doctr's Wife, and
And I did mention during my awards speech that only a madman or a fool would tempt fate by doing it again, having won. And that I am now on my third draft...
Bleeding Cool suggests that the episode may be intended for the second half of the just-started series...are they really planning to broadcast episodes early next year that haven't even started filming yet?
If so, that would certainly support my supposition that, even if Clara hadn't originally been planned to be connected to Oswin, there was plenty of time for rewrites to make that connection.
EDIT: Fellow Hugo winner
Charlie Jane Anders confirms that Gaiman says his episode is slated for the second half of season 7. So, while that doesn't mean it WILL be part of the second half of season 7, it would certainly suggest that yes, the current season, and specifically Clara's arc, is still being written.