I vote for Matt Damon as Kirk and Ben Affleck as Spock. Boyo, can you imagine their first kiss?!!!?
Okay, enough of that silly dreaming...I seriously see nothing wrong with Matt Damon as Kirk...I'm sure he'll make an excellent dream boat captain kissing all the aliens and letting his hands roam were no man has gone before? What else is required besides an ability to speak in stilted sentences?