The Observation Deck

The wife and kids have been hounding me about the idea of building a small treehouse in the backyard. Has anyone here done this before? Do you need to get a permit? Get it inspected? Do the same codes apply for treehouses as decks?

The DIY Network had a cool special on treehouses.

I thought I read somewhere that you had to have a permit for a free standing tree house, but not one that's actually in a tree. I can't seem to find where I read that though!