Not all PS3s are backwards compatable. As far as I know it's only the 60gig version. It's the reason I stick with it even though it's the most unreliable model (I'm on my third).
I don't go back to too many of my PS2 games, just the three Silent Hill games that came out on it and Forbidden Siren 1 and 2. I tend to go further back to the PS1 titles: Hogs of War, Warzone 2100 and Re-Loaded to name a few.