Here are some of the latest casting rumors,
Mike Vogel (Kurt Russells son in Poseidon) is in the running for the part of James Kirk.
Karl Urban (LOTR) is up for an undetermined part (McCoy?).
The only parts that are actually cast (that have been announced anyway) Uhura (Zoe Saldana), Chekov (Anton Yelchin), young Spock (Zachary Quinto), and old Spock (Leonard Nimoy).
The word is they are looking for big time star to play the villian (rumor is Russell Crowe), but they are looking for someone who does good box office in the overseas market.
There are a bunch of maybe spoilers going around about the plot, but I won't mention anything here other than Spock looks to be the main character in the film.
If want to follow this film, the best place I've found is
http://trekmovie.com/