PSN have gotten a lot better with PSone classics. For me I'm sold, as those were the majority of games I played growing up. Every time the subject of backwards compatibility comes up people are usually referring to the PS3's lack of PS2 compatibility. I know it's possible for them to implement, but I actually read somewhere that they didn't in the newer versions because of compatibility issues. The system has to basically emulate the games in order to play them. Emulating PSone's is laughably easy and takes a small amount of power and little software and hardware maintenance. Since it was easy to implement and was cost efficient, Sony included it on all models of the PS3. Now, I don't know the details, but I am aware that the older versions of the PS3 that do have BC had many falts and some games had many bugs. This is most likely due to the fact that the PS2 is a much more powerful system (which makes sense because emulating a PS2 game with a constant 60fps takes a very powerful PC even today - I'm talking Quad Cores to get the most out of the experience). With the PS3 slim out and about, it makes sense that Sony would cut some corners and completely remove BC of PS2 games in order to save tons of money.
That's not saying I wouldn't love being able to download PS2 games off of PSN, that would be awesome. That's why this Christmas I invested in a new PS2. Makes things a lot easier. Not only that, but most games are dirt cheap everywhere you go, and they would probably be more if they were on sale on PSN. Might as well enjoy it while it lasts.