Yeah there seems to be rumors about backwards compatibility issues with several PS2/PS1 games. On the bright side, Sony aims to fix all these issues with a firmware upgrade before the launch on friday, so it shouldn't be too major a problem.
As many as they can, the Guitar Hero bug wont be fixable... at least by firmware updates.
In all honesty, I own like 2 PS1 games and as long as they both work (or more importantly just FF VII) then I could care less. There are no PS2 games I care enough about to be saddened if they wont run on the PS3. I did however want to get MK Armageddon as that looks sweet.
I've checked out Tales of the Abyss (JAP), FF12 (JAP), KH2(USA). However, the US games don't play on the JAP ps3. The Japanese PS3 is still region locked for ps1 and ps2 games, and some of the odd japanese ps2 games play without sound. However some of these games are fixed by not using an hdmi cable... However, some of the games also don't work. Sony plans on fixing some of these problems in a later update. anyway, thats the situation.