It appears as if all the DLC going forward for Rock Band will no longer work Rock Band 1 & 2. A Harmonix Representative told Joystiq that they will not work “due to the new authoring standards, inclusion of new instruments, new lighting / venue cues, etc.”
All DLC put up from October 26 & on will be Rock Band 3 only, however if you have RB3, you can still get any of the past 3 years of DLC. Also, all Rock Band Network songs will be able to be downloaded to all 3 games.
Credit: Joystiq