Talk about rotten tomatoes. A thread over at AVS Forum reports that some users are finding mold-like spots appearing on some blu ray discs. These spots, described by many as "disc rot", render the discs virtually unplayable. Users are reporting that the blemishes are appearing beneath the top surface layer, which leads many to believe it's a problem with the coating.
We're not sure how widespread this is, or if it's a flaw in a particular producer. But definitely something to keep an eye on if you've upgraded to Blu Ray.