Yep, its been done. I think they even sell kits to do it.
If you're real interested in home efficiency, I suggest you check out the
EcoRenovator Forum. Its EcoModder's sister site for home efficiency. We even have a guy who is doing a
DIY geothermal system. His target is to stay under $2k and he is doing it so far. He has the loop field in and is working on the heat pump itself.