Which is surprising, because it means that she'll either be appearing alongside Jean Grey for the first time in any medium since Jeff Parker's amazing Exiles (and Grant Morrison's New X-Men, of course) or appear in place of Jean. If the movie sticks to the continuity of the first trilogy of X-Men films, then we'd expect Beast, Jean Grey, Cyclops and Storm to make up the main cast of the film, but the appearance of Emma Frost knocks things for six, a little. Sure, Emma appeared in Wolverine: Origins, but that film was dreadful and we were hoping it'd been struck from memory.
The film now has four cast members signed on: James McAvoy is playing Xavier - odd - while Michael Fassbender will be Magneto. Beast will appear, played by Benjamin Walker. So at least we now have some idea which direction the film is going to take in terms of storyline: Mags and Xavy teaming up then splitting apart because of their conflicting ideology. But where will Emma Frost place in all of this? As she doesn't appear in the later films, it might be that she gets killed off or removed from the playing board somehow, but what if the Hellfire Club turn up? Could we expect Sebastian Shaw and Mastermind to appear? And if so, what might that mean for the first appearance of out beloved Dazzler, and Sage? The casting really does open things up in terms of storyline - anything could spring out of this.
Interestingly, it also appears as though Banshee may turn up at some point. What role will he play, then? Are we about to see Marvel set up a Generation X movie? That would surely be the best-case scenario for the movies. Mystique also seems set to appear at some point, and Marvel wouldn't be Marvel if they didn't try to shoehorn Wolverine into the movie somewhere. It's all very interesting at the moment!
Oh, we forgot to mention: Emma Frost will be played by Alice Eve.