According to JoBlo.com Zachary Quinto (MR. Spock) wants a break from Star Trek. "I’m stepping away from Star Trek for at least two years — no movies, no conventions, no anything for a while," said Quinto.
He went on to say “I’m stepping away from that genre and exploring a lot of other territory."
It's not clear at this time what kind of effect this "leave of absence" could have on the June 29th 2012 release date for the next Star Trek.