If you have a lot of dough, the TreeHouse company can build you one super awesome tree house.
This one's called the Falcon Project. From the website:
"At the end of a long driveway in the grounds of a grand Scottish estate in Fife, stands a 500-year-old cedar tree. After suffering a severe lightning strike that destroyed the largest of the tree limbs, it looked a sorry sight next to the magnificent copper beech that guards the gateway to the grounds.
Our client's brief was to fill the gap left by the missing cedar tree trunk with a fantasy TreeHouse for his children and to create a TreeDeck under the shelter of the Copper Beech canopy for the adults, so they could relax or entertain while watching the children play. "
http://www.treehouse-company.com/index.cfm (Doesn't work properly in my Firefox, but you can try IE)