To me, there are so many problems with this outsider label: on the one hand no one wants to call it "retarded/crazy person/homeless/paranoid kook art."
I find mimi smartypants consistently funny, but this one about insider art (and other things) particularly struck me. Maybe it's because I haven't been to her site in a week or so.
In case you were wondering, here's your answer: What is Outsider Art?
And, for the sake of completeness, you might check out Intuit or Visionary Art, both sites devoted to Outsider Art.
UPDATE: Diesel Sweeties gets on the crazy-person art bandwagon.