Havent played with a PW yet, but watching this a few days ago:
EEVBlog : PW Teardown
I'd think that tapping or uneven pressure on the surface could lead to the layer bending locally, which could result in differences in the refractive quality and perhaps that pink bleed that has been commented on. As well as the obvious light/dark spots.
Pretty interesting that there's also a magnetic switch on there too, new covers?