To make it cheaper to manufacture, they got rid of the PS2 chip that was originally in all of them. In Europe and now here with the newer PS3 models they're just doing software emulation, which isn't quite as good and not all games work with it.

... but basically they want people to buy PS3s and play PS3 games, not just use it as some magical PS2 machine XD

It's entirely possible that they will go back to supporting PS2 games with firmware updates in the future to the new models without backwards compatibility, but who knows~

Thanks for the congrats, I think, my wallet isn't very happy right now though XD