Step 15: Enjoy!
The treehouse is a great little (36 sq. ft.) haven for the kids; they love it and so do all their friends. It's just big enough for two 2' wide air mattresses, so it's perfect for a summer sleep-out.
Update 2013: I'm happy to report the treehouse is four years old now, it's suffered no damage from windstorms, snow, or tree growth, the trees are healthy, and it still gets lots of use. We've made a few updates over the years, adding a pirate's treasure chest, a swing, a braided climbing rope, a BEWARE sign we use out front at Halloween, and it's well-equipped with Nerf guns & water pistols. It's just big enough for two single air mattresses, so it's fun to sleep out in, too.
Update 2014: Growth got to the point that I took the treehouse down for safety reasons (the tree started to open some joints). I am in the process of updating the instructable to reflect the rebuilding process, which is mostly complete now.