When the Avengers vs. X-Men crossover began, Marvel swore up and down that Jean Grey would not be resurrected during the event, despite the presence of the Phoenix force in the storyline (this thing had a storyline?) However, judging by the cover of the final issue of the A vs. X series, Marvel is a big fat bunch of liars whose pants are clearly on fire.
That certainly looks like Jean in her “Dark Phoenix” persona, but, as many have mentioned, it could also be Hope — the red-haired young girl who everyone in this crossover is fighting to contain/protect/whatever, or Wanda Maximoff, aka The Scarlet Witch. A vs. X #12 hits comic shop shelves in October, and will lead directly into a new team book called Uncanny Avengers, which features members of both the X-Men and The Avengers. I’ve included an image of that cover as well.