From what I heard you have to use many bones or morph targets in Unity right now (just search in their forums to get some code / ideas). One may hope for an upcoming update - in their forum they asked for ideas about the animation system and how it should be improved so there might be some updates on this topic with an upcoming release (LINK)...

The source engine uses some sort of special animation system and thus no standard bones or something like that. They've also extended that system over time making it a quite complex thing which sort of simulates a face's muscles...