Ray Rhamey at Flogging the Quill has a great post up on his blog about this topic. I've struggled mightily with decisions about how much to describe and how.
I've come to the conclusion that although Ray's post is right on the nose for the kind of fiction he writes (mostly suspense), the question of how much to describe does differ among genres. Romance seems to almost require detailed physical descriptions of main characters. Science fiction and fantasy require some description as well, since the characters are expected to be physically unusual in some way. Other genres can leave more to the imagination, since they are more obviously ordinary humans except for some distinguishing characteristic like a unibrow or flaming red hair.
What do you think? How do you handle the problem of describing point-of-view characters? Or do you prefer to leave it up to the reader? How much description do you think is expected in your genre?