We tested the new functionality with an early preview version of SingStar Vol. 2 and it works just as you'd expect. When you're in the song carousel, hit SELECT and you'll be able to insert a new disc. Placing SingStar in the drive brings back ye old song list. Placing a PS2 disc in the drive will simply result in an error message.
We're eagerly awaiting the eventual patch will allow PS2 games to be used in the PS3 SingStar games. That's exactly how we'd like to enjoy games like the PS2-exclusive SingStar Country.
[Thanks, Joe B.!]