I don't know (or frankly care) how it is implemented. While the UI is flat, the apps use transitions and a parallax effect to build the illusion of the depth (similar to how Apple does it using their gyroscopic parallax effect on the home screen).
There is no depth or layering in metro, not even small drop shadows and definitely not layered application screens. The panaroma metaphor is panning left or right with some disjoint movement of the background (not really parallax, very fake) and the cutting off lots of text...
Lets wait until we can judge them side by side. I have a windows phone device, but no apps (my market doesn't really have anything decent), so I'm not really sure if the metaphors have evolved much from the panaromas.
