Let's face it: Patrick Stewart IS Professor Charles Xavier. Right up there with Robert Downey Jr. as Tony Stark, it was great casting. Now it looks like he's coming back.
Stewart himself confirmed, "yes, I'll be reprising," but it's unclear at the moment if he's referring to the current series of X-Men prequels, or a new fourth film in the original Bryan Singer trilogy. It's entirely possible that the upcoming Days of Future Past will combine the two, creating a time travel paradox that negates the entirety of X-Men 3. Maybe.