Source for sample designs for a Tree House?
A good design would be what @Tester101 suggested.  Build a platform around the tree.  On one side of the platform could be the tree house.    Some examples are here:

http://www.buildeazy.com/plan_links_treehouses.html

There's a good picture of platform around a tree, now just imagine a bigger platform with a tree house on one side of it.