Okay I came across this while downgrading to several PlayStation 3 firmwares, here is how to play PS2 Games on a non backwards compatible PS3 in six simple steps!
Step 1. Enter Service Mode.
Step 2. Downgrade your PS3 System to System Software Version 2.50 (this is the one that supports more PS2 gaming).
Step 3. After downgrading don't get out of service mode.
Step 4. Uncheck Game-auto start in System Settings.
Step 5. Insert your PS2 disk cd/dvd on PS3.
Step 6. Load it and do not press the PS Button while playing otherwise it will crash.
Tested on a CECHP01 Console. The one game that would play exactly like in a PS2 was Twisted Metal Black.
Note: DO NOT TRY THIS ON A PS3 SLIM IT BRICKS! (But can be recovered while in service mode) This can only be done on a PS3 System bought 11/20/2008 80 GB PS3 12/31/2008 160 GB Uncharted Bundle (With Pain Voucher)
Cheers and happy playing.