Anywho, treehouses have come a long way since I was a kid and seem to be quite in fashion for adults. Check out the treehouse pictured above, designed by Kobayashi Takashi
The treehouse in this one above is a module by this company - www.freespiritspheres.com Cool but spendy.
A great resource and website about treehouses is http://http://www.treehouseworkshop.com/ They have basic guidelines and instructions for building a treehouse, design custom houses, and one of the fellows - Pete Nelson - has written a couple of books called Treehouses of the World and New Treehouses of the World.
And, most exciting, did you know that there are treehouses you can stay in while on vacation? The one below is Fernie Caste Treehouse in Scotland. You can find a listing of places here: