One time when i was starting a world i came across a floating island i the Jungle. The island is a small one layer dirt island it two jungle trees on it.I am not and to prove it here is the seed: 242207216
This seed only works with a Minecraft 1.4.6, you will might spawn in the ground, just dig up and turn and you will see the floating island. Don't you just love generating errors
I am also working on a new mod and will be out soon.

-Ben