I'm loving Munn as Psylocke, This is great news. And she's not even an unknown actress.
The fact that she's mixed race, a comic geek, used to cosplay (eg. Chun-Li and Wonder woman) and can act really makes me excited.
I'm not holding my breath that she'll get a big portion of the movie, tho Psylocke as one of the mutants-turned to Horsemen would be a great plot for her and the future X-Force movie.