The New York Times has a great article by Atul Gawande about itch, and how sensors can go wrong. It's not the same as persistent pain, but it sounds awfully similar in behavior. Here is a link to The Itch. Here is a link to an audio interview about it.
Here is Jonah Lehrer's blogpost about it in his Frontal Cortex blog.
Here is a link to Atul Gawande's website.