Well, I ended up making, not a treehouse, but a forest scene. I haven't (yet) put platforms in the tree branches, because I love the flexibility of having the forest setting. It can be a home for animals, a base for a nature table, and a setting for fairy tales -- more open-ended than a regular treehouse playset, imo.
I have been talking to dd10 about building treehouse platforms together out of wood scraps and cloth or twine stiffened with glue. We are planning to make them removable. In the meantime, we're enjoying the play forest setting the way it is.
We made it out of wood from overloaded branches that broke off our apple tree last summer, so I'm calling it our "Giving Tree." Spent a total of $.79 on it, buying screws. Dh reminded me after that we had screws in the shed that would have done. I'll try to be more frugal next time, honey!

Here is the basic scene, branches screwed and glued to a base of wood and burlap. I wrapped the pointy ends of the branches in burlap for safety, and painted white glue on the burlap base to give it strength and protect it from snags:
http://i182.photobucket.com/albums/x...eehouse5-1.jpg
The tree became a home to dinos and giraffes....
http://s182.photobucket.com/albums/x...eehouse1-1.jpg
A green playsilk made a forest canopy and Plan Toys dollhouse dolls work really well in this setting:
http://s182.photobucket.com/albums/x...eehouse3-1.jpghttp://s182.photobucket.com/albums/x...eehouse2-1.jpghttp://s182.photobucket.com/albums/x...treehouse6.jpg
We added some animal models and a basket house for grandma to make a Little Red Riding Hood playset:
http://s182.photobucket.com/albums/x...treehouse7.jpg