Rock Band DLC released after October 26, 2010, is Rock Band 3 specific and will not be compatible with previous versions of the platform. This rule applies to all future Rock Band DLC, whether it's labeled "RB3 version" or not. Songs released before that cutoff date will remain available and compatible with the first two Rock Band titles. Also, new RBN tracks will be compatible with all versions.
A Harmonix representative explained to Joystiq that new tracks won't be backwards compatible "due to the new authoring standards, inclusion of new instruments, new lighting / venue cues, etc."
Although it's a bummer that owners of earlier Rock Band games won't be able to play new tracks, they won't easily be able to purchase the tracks by accident. Any songs released after 10/26 will not show up in the Rock Band and Rock Band 2 in-game store, nor will they show up in the standalone music store app or under RB1 or RB2 on the console's marketplace.