The alternative is to get something out before the new hardware launches, but somehow I think we'd have heard something by now (it's almost been 2 years since RB3 launched.)
As a question of technical capability (and not of feasability, cost effectiveness, or any of the other things www has already thought of to try to tear down my post), can downloadable hard-drive-installed games read data from game discs, on the PS3 and the Xbox360?
I just don't know if installed games can do that - if an installed game can invoke disc drive functions. Theoretically, it would be... amusing... if (a patched) Rock Band Blitz could detect the presence of an RB3 disc in the drive and could stream the music data files from their known location in the filesystem on the disc at run-time, allowing the RB3 tracks to be played by RB3 owners without the mess of a formal disc export. (Similar think could be done for the track files embedded in the Game-on-Demand install). And I wonder if not being 'exported' or downloaded would ease any issues related to licensing costs normally associated with an export.
But yeah, it would be really cool to play the ultimate driving song in Blitz......but that'll have to wait. Possibly forever.
I really wanna play 25 or 6 to 4 in Blitz.
Tubbs has a new home
I would like to the play The Cure's song. but that's really the only disc song i ever touch anymore. the rest are meh.
The Cure 
My Chem. 
The point it, the new export could open up more achievements, and the money they earn from the exports could go towards making a new Rock Band, even if that means another track pack. Or maybe even another band based game. The Beatles Rock Band and Green Day Rock Band were both huge successes. Guitar Hero did Aerosmith, Metallica. Those are all big bands, but if Rock Band did a band like Pink Floyd or another over requested band that has never had a song in Rock Band or Guitar Hero, let alone have pro mode in it, they would simply conquer all music and rhythm games out there.
Think about it guys, it could open up a whole new door and start something huge. Agree to disagree?
SillyStou: What will happen to this place when Rock Band ceases to exist?
RockBandRocker: We will all migrate to the Dance Central forums. It will be like Moses leading the Israelites out of Egypt.
Munnchy: Only Flashier, with impeccable dance moves.
RockBandRocker: With more glitter and glam. We will survive