It seems that despite statements to the contrary prior to launch, the PS3 is having difficulty playing the entire library of PS2 games. Various compatibility issues are rendering certain titles unplayable, which is strange considering that the PS3 has the two core components of the PS2 built in to prevent precisely that. Sony is aware of it, but doesn't have a planned fix at the moment:

Sony announced this problem on its Japanese Web site on November 11, and said it expects similar compatibility problems to remain for the North American launch on November 17. According to Sony Computer Entertainment spokesman Satoshi Fukuoka, a select number of the 8,000 PS2 games are failing to work properly on the PS3, some that couldn't be played and others making the wrong sounds or images.
There's a short list in the article that lists some of the affected games so far, and Sony has stated that a future fix may include software patches or system upgrades.