Sony has released a free online patch that allows the PlayStation 3 to play PlayStation 2 SingStar discs.

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PS3 owners can now expand their SingStar PS3 library to include tracks from the likes of SingStar Pop, SingStar 80s, SingStar Rocks!, SingStar R&B, SingStar Amped and other titles from this musical canon.
 
All a user has to do is to:  

1. Ensure your PlayStation 3 console is connected to the PlayStation Network and insert the PS3 SingStar disc of your choice. 
2.  As the PS3 disc loads, you will automatically be prompted to download a quick software update.  Once it’s downloaded, you’re good to go 
3.  Start PS3 SingStar session as normal – then head to the carousel, hit the ‘Select’ button and you’ll be able to swap it for any PS2 disc for the rest of the session 
4.  Next time around, just repeat the process and start again

See: www.singstargame.com