I can't resist throwing something into the mix here. I have seen articles discussing a NASA project for high speed helicopters. I believe the concept is called X Wing. http://en.wikipedia.org/wiki/Sikorsky_S-72

The rotor blades stop in flight and become wings. This means that the blade on the retreating side reverses direction. They accomplish this by using an oval shaped blade and blowing air out of the trailing edge to form a virtual airfoil.

I know that this is much to complex for RC. Just thought I'd help with the brainstorming.

Interesting topic.

edited to add this link to a UAV that uses this technology http://www.answers.com/topic/boeing-x-50