Everyone and their mothers said:
Notch is just stealing this idea from a mod.
Actually, Notch beat the modders this time around in the idea-race. InDev had map types, which changed the way the generator worked, and one of those types was floating. He removed it ages ago because it was buggy, occasionally spawned you on a floating island of sand or gravel (which made it impossible to get off, since putting anything by it or digging from it would cause the island to fall) and there wasn't any good way to make the generator work in InfDev (plus the obvious fact that map types are no longer in use). Most mods are attempts at recreating this (though personally, I prefer the skylands downloadable map, because it's much more awesome) so he's really probably building this code off of his old code, rather than from someone's mod.