Update: Slight change of plans; according to MTV Multiplayer, Sony isn't allowing you to export the music. Rather, there is an option to swap discs -- by pressing select during a session of SingStar on the PS3 and exchanging the disc for any PS2 SingStar game, you will be allowed to choose the songs from the PS2 disc. This is not permanent and you will need to have the PS2 disc in the drive in order to sing these songs.
Moreover, as one of our readers asked, there were those of you who were concerned that PS3s without backwards compatibility would lack this feature. No need to worry; Sony has stated a free patch is coming for current SingStar game and this new disc-swapping feature will now be standard on all future incarnations of the title.