I woke my PSP out of sleep mode and saw a mark on the screen. It wasn't dirt, and it wasn't a group of bad pixels because when I moved the XMB up and down, the smudge moved too.
Movie and pic here: http://www.megaupload.com/?d=12MK0WM8
I put my PSP back into sleep mode, woke it up, and the smudge was gone. I have no idea how I triggered it...