Not too sure about that... I know my original 60GB version had backwards compatability (to a certain degree - thanks Sony), but the 40GB version, the compatability was dropped...
Not sure exactly how the new models rate...
I think all PS3 can play PS1 games. The 40 GB, new 80GB and 160GB can't play PS2 games.
Otherwise, it would piss some people off if they bought a PS1 game from the PSN without knowing it wouldn't work on their PS3.