Harmonix has announced that all downloadable content for Rock Band released after October 26, 2010 will only be compatible with the third instalment in the music simulation franchise.
Apparently, future DLC won’t be backwards compatible with Rock Band 1 & 2 "due to the new authoring standards, inclusion of new instruments, new lighting / venue cues, etc".
Previous DLC will still work with earlier titles in the series, as will all previous and future endeavours on the Rock Band Network.