Hey guys, as far as I can tell, all PS3's can play PSX games (that is, PS1).
However, not all can play PS2 games. that's the key difference. The old 60GB ones with the Emotion Chip & stuff could play 'em, but as the company started taking parts out of the PS3's to reduce costs, PS2 backwards compatibility went out the window.
Not sure if/when it will ever come back, but if it does, it'd be via PS2 emulation in the PS3, not via a chip.