ASUS Response: We've received a response from ASUS that they're unable to replicate the problem on production units (I/O units are, technically, pre-production), so this may have been sorted out in time for the initial consumer run. It appears, then, that this problem is at least affecting only some units.
Update: Here are some more comparison photos from a different Nexus 7.
Some have suggested this is a native behavior with IPS displays called "image retention." I did a side-by-side of the same image with a Transformer Prime and Trasformer Pad 300 - both of which have IPS displays.