Not sure which yet, but either the BD Drive on my new-to-me PS3 is about to go out or the disc for the used game I bought with it is damaged. Fortunately, with these 'George Foreman' PS3s repairs are simple and the BD drive is very easy to get access to, so I'll probably keep it even if it gives up within my 30-day warranty.
Seems to work great with all the game discs I've had forever, so it must just be the used one I bought isn't in great shape.