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...