I happened to come across a web site -- "My Travel Clinic (Inflight problems)" There was a comment about head lice -- "occassionally head lice and other skin parasites may be passed on through contact with airplane seats where previous passengers have been infested. It must be stated though, that most airlines carry out thorough cleaning of the cabin and other facilities between flights."
Have you ever seen a Southwest flight stop the passengers from entering while the cleaning xcrew is scrubbing down the seats? Probably not.
Try using TravelKleen(reusable)headrest protectors -- a very easy way to keep your hair clean and, best of all, the chances of getting head lice from someone else is greatly minimized.