Early adaptors of the PlayStation 3 were lucky enough to be able to play PlayStation 2 games alongside their PlayStation 3 games. However, when the new SKUs were announced, the Emotion Engine, which allowed for PS2 games to play, was removed from the console to cut costs.
However, PS2 emulation for all consoles may not be too far off, because Sony Computer Entertainment America have recently filed a patent for technology allowing the Cell Processor to emulate the Emotion Engine allowing all PlayStation 3 owners to play PS2 games.
Definitely exciting. Now when will this firmware update hit our consoles?