Out-of-body experiences have been theorized to have much to do with an area of the brain known as the angular gyrus. In tests, they can stimulate nerves in this area and subjects will report that they feel like they've left their body and have floated up to the ceiling. Supposedly, this small cluster of neurons is what "centers you in yourself".
I think this section of the brain also has something to do with a really interesting disease known as
Cotard's Syndrome. Along with feeling outside of yourself, you can also feel that your grasp of reality and "frame of reference" are so far off that you believe you are dead or no longer exist.
Freaky.