It's in Sony's interest to keep the PS2 brand strong. No one buying a PS2 Singstar bundle is even concidering the PS3 at this point anyway. It doesn't hurt the Wii for the simple fact that the Wii will continue to sell out, and a few people deciding on a PS2 instead isn't going to change that.
I think an ever better effort would be for Sony to just drop the PS2 to $100 this holidays. Then I think you'd see sales at least on par with the 360; and I think in particular it would do well as a system parents will buy for their kids, expecially given most of them won't be able to find a Wii anyway. The PS2 in a sense could be the consilitory system for those casuals and non gamers that can't find a Wii, if they aren't one of the tens of millions who already own the system.