Hollywood Reporter is reporting the addition of Meiling (Mei) Melancon to the X-Men 3 cast as Psylocke. Melancon worked previously on Rush Hour 2 with director Brett Ratner. While she has appeared in various incarnations in Marvel's plethera of X-Books she will fight against the X-men in the film, as a member of Magneto's Brotherhood of Mutants.
Considering that this film is due in theatres in less than five months, I'm suprised that they're still announcing new cast and characters. I'm also supprised that so much fuss is being made over an actress with so little credits. Check out her IMBD listing to see what I mean.