At a Dragon Con 2009 panel discussion, Leonard Nimoy revealed that he will not appear in the follow-up to J.J. Abrams' Star Trek. Appearing with William Shatner, Nimoy also took the opportunity to razz Shatner about his apparent disinterest in the movie.
There are no plans for me to return for the second movie. I think the Spock character is very well established as portrayed by Zachary Quinto. And I think if you saw the movie Bill, you'd say the same of Chris Pine.
Shatner's response? A curt "bulls**t."