For those who haven't heard:
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BBC announcement
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Outpost Gallifrey report
At least we already got to see the actor in "Army of Ghosts", so she's not a complete mystery. Interestingly enough, Martha is not supposed to debut until the first ep of season 3, which I guess will therefore probably be a character intro ep like "Rose".
So the 2006 Christmas special (which they've already started filming) will feature a "guest" companion, possibly introduced in today's episode. I guess we'll know in a few hours.
It's gonna be a long wait until Christmas and next spring.