First things first: the LCD is still fully functional, no broken pixels or the "shattered LCD" bug in TF3d. The "distortion" does not appear under all lighting conditions --- it's actually impossible to see in areas with high contrast lighting (ie: lots of bright and dark areas such as daylight or most rooms), but becomes quite visible in flat/even lighting (like a bathroom or narrow hallway).
The distortion shifts in response to relatively heavy pressure, but always returns to the same "shape" as seen in the picture.
I am using the included screen protector, but I have removed it to confirm it is a problem with the LCD and not the protector.
I'm tempted to say that this has happened suddenly, but given that I've been using my TPro in anything but flat/even lighting conditions, I can't say for sure; I only happened to spot it by chance today. And just to be clear, I haven't dropped the unit, gotten it wet, or anything drastic like that. It's an expensive gadget! (And all the more reason something like this bugs me )
For reference, my unit is: