A demonstration scene of the floating island technique. Let me know if anything is unclear.
It uses the Sweet Birch Xfrog tree model available [links expired].
Let me know if this is helpful, I've tried to make the nodes fairly self explanatory.
One point of note is that "Cast Shadows" has to be turned off on the plane model as the masking of the plane dose not mask the shadow, so the shadow is projected as a full square shadow, not the shape of the island!
Enjoy
Richard