An interview with Jenna-Louise Coleman in "The Evening Standard" has stated that she is confirmed for the 2013 Christmas Special together with Series 8. They say:
It has just been announced that the 50th anniversary episode will be in 3D, and Jenna-Louise’s multiple Clara Oswin Oswalds have been confirmed for the 2013 Christmas special and series eight.The full interview is here and it's not spoilery. She talks about her family, her career and, of course, "Doctor Who":
‘I didn’t really know anything about Doctor Who but Steven really liked that. It meant that when I went into the audition with Matt Smith I could be more spontaneous because I didn’t know him as the Doctor. And the plot seemed to evolve from audition to audition, with more scenes being written, and characters being introduced. There were times when I thought they had no idea what they wanted.’Interestingly, the article appears to confirm Matt's involvement in the Christmas special too:
She has just found out that this year she will have a three-month break from filming Doctor Who when Matt Smith goes to America to star in Ryan Gosling’s directorial debut How to Catch a Monster and was immediately on the phone to her agent, keen to start setting up meetings to find something to fill the slot. ‘They said, “Just calm down, Jenna.” ’ But I don’t think that is possible for her, not in this quantum reality.Sounds good to me!
Thanks to Ruther2 for the heads up.