They've cast young Spock (read article here) It's Zachary Quinto (Sylar) from Heroes. Leonard Nimoy is also coming back to be in the movie in some capacity. William Shatner MAY be back at this point. They still have not cast young Kirk yet, but I read in some articles that Matt Damon has said he is too old to play Kirk. I am glad as I thought he was too old too! :-)
What do you think of the casting? I think Quinto looks a lot like Nimoy, and hopefully he'll be able to portray Spock as well or at least almost as well as Nimoy.