The Nintendo Wii finally got its own version of Rock Band 2, but the 30 downloadable songs that Harmonix promised would be available at launch are nowhere to be seen. Not to worry, their official forums state that "Harmonix is working as fast as possible to deliver the 20 free bonus songs for Rock Band 2, plus an additional 30 songs from the Rock Band DLC Catalog to fans as early in 2009 as possible."
Video game rockers with the Wii version will be able to purchase and download the entire back catalogue of Rock Band DLC, which in total numbers in the hundreds of songs. No pricing has been announced, but I suspect it will be similar to the price on other consoles at around 2 bucks a song. Rock on with your bad self, Wii. You've earned it.
