The Treehouses are an old Disney Resort. They were their own resort originally, then were lumped in with the Disney Institute when that was operating. Much of the lodging at the DI has been leveled and is being replaced by the new DVC resort Saratoga Springs. The treehouses have not been operational for some time and are in limbo due to environmental regs, or some other type of red tape. A decision on what to do with them has not been made (or at least not announced).
They were really great places to stay. There were 2 or 3 levels, with a full living room and kitchen, 2 or 3 bedrooms, and 1 or 2 bathrooms. My DH's family stayed in them when he was young (early 80"s) and since we started going to WDW in the last 8 years, has asked to stay there when they join us. But, they've been closed since they started tagging along, so I've never seen them myself. There are lots of folks who hope they restore them and put them back to use!
Katie