Actually, I think Matthew has a point when he distinguishes unschooling from attending Sudbury Valley School. I love John Holt's books just as much as the next person, but most of the unschoolers I know are getting a much different kind of education from what students get at Sudbury Valley School. Methink they doth protest too much. Few parents will allow younger children to be unsupervised with a diverse group of other children of various ages. Even at park days, lots of parents are standing by to intervene in case of problems.
Parents also limit whose houses their children can go to, and when and how often.
These generalizations do not apply to every unschooler, but they do apply to most of the ones I know.