Just a note to Harmonix if they ever do do this, since Nord mentioned they might look into it:
The DLC needs to be cross-compatible with PS3, or I'll imagine you'll lose a whole lot of sales. The major turn off for Unplugged for me was that none of the DLC I had would work on the game, and it was still full price to get thr Unplugged versions of songs I already had.
The always online issue would cause problems with portability and the Vita. That and re-certifying all the DLC would be costly. Not likely to happen.