That's actually a pretty good question. I've always called it the "lipshrug" in my head (and hey, if "bitchface" has become a valid term...).

I've only ever described it once in my writing. I think it went something like this:

The corners of his lips quirked down as he considered the possibility. "Huh."

*pokes at sentence* Blech.

It just comes back to the "Huh.", doesn't it?