I believe it is still unibody, but on the other of the phone instead of the normal half everyone expected it to be.
Its like helmet now. The unibody is removable, but it still basically covers the whole phone once it is installed.

Some people might dislike it, but honestly, after the whole DHD problem with its stupid flimsy side doors, this is much more functional in the long run. It just sacrifices a little bit of sturdiness due to it is no longer fixed but removable (still much stronger than what you can expect from SGS2).