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.
Update May 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 October 2013: Growth got to the point that I took the treehouse down for safety reasons. The tree started to open some joints, cracks had appeared in the main structural supports, and the washers were embedded in the wood. Amazingly enough, I was able to remove all the lag screws, so when I rebuild I can reuse the same holes (and all the wood except for the 2x8s).
Update July 2014: The treehouse has been fully rebuilt. The instructable has been rewritten to reflect the rebuilding process, and there are a mixture of old and new photos throughout. Main changes: walls no longer have gaps, entry deck has been embiggened, and longer, heavier duty lag screws were used to give more room for the tree to grow. I anticipate it lasting at least another 5 years before taking it down again (probably permanently).