I'm no fan of DLC, but at least this exposes Harmonix as the liars they are.
The reason Rock Band "couldn't" have DLC is because the graphics in the PS2/Wii version are FMV. So, yeah, technically DLC is out of the question when doing so would require you to download a big ass video file. A lazy port of a lazy port.
I think Harmonix may have ceded the music game market on Wii to Activision because of this. Since the controllers aren't compatible between the two games, I don't see many people willing to buy two sets of expensive redundant peripherals, and Activision has a big head start with millions sold on the Wii. Even if they actually put some effort into Rock Band 2, it will be too little, too late.