They are each being seriously considered for the key roles of Captain Kirk, Mr. Spock and Dr. McCoy, respectively.
Lt. Sulu is rumored to be played by Daniel Dae Kim, who plays Jin Kwon on the popular TV show, Lost.
Tom Cruise is said to be playing the main antagonist as himself: an alien bent on world domination. David Beckham and Posh Spice are his henchman, and the Cruise triumvirate share an uneasy alliance with John Travolta's mysterious OT-8 band of warriors.
OK, seriously, Tom is really just going to be starring in his tribute movie to L. Ron Hubbard.
Seriously. It's called The Thetan, and Posh Spice really is rumored to play the role of the alien bride. Apparently major studios rejected it (no surprise there), so Tom is financing it himself. Did he learn from Travolta's mistake with Battlefield Earth? One of my favorite movie reviewer quotes from that one:
The script, based on a novel by Scientology founder L. Ron Hubbard, is deeply dumb, depressingly derivative (ripping off "Planet of the Apes" the most) and just plain nonsense.
[USA Today Andy Seiler]