Here’s a set of very nicely done portraits of the model/actresses that play the bosses in MGS4:
MGS4 Bosses – a photoset on Flickr

Hi-res images are here: GGL Wire

They are known as the Beauty and the Beast Unit, and possibly the Beast Corps. I don’t think these models are voice actors, but I read that they are the mocap actresses. I’m (obviously) trying to learn as little about the game as possible before I play it, but you can read more about these dealy beauties at Joystiq and AssociatedContent.