Don't know if its correct but I heard a whisper that the PS3 no longer plays PS2 games.
The 20GB, now phased out, has the hardware Emotion Engine and can play the PS2 games without a hitch.
The 60GB (the first US one), also phased out, also has the hardware Emotion Engine.
The 60GB (PAL one), also phased out, has software emulation with problems with a minority of games.
The 80GB, soon to be phased out, also has software emulation.
The 40GB has no hardware and no software emulation to run PS2 games.
read more here:
http://en.wikipedia.org/wiki/PlayStation_3