When the official announcement finally came last week that Rock Band 2 would release today on Xbox 360, one little bullet point on the press release about an additional 20 free DLC songs nagged Joystiq. Looking at the back of the Rock Band 2 manual now, they still have no idea what the DLC is, but they do know how they're going to get it. Going to the site listed in the image above currently gives this message: "Please check back in a few days for the opportunity to enter your Registration Code and sign up to receive the Bonus Downloadable Tracks." Joystiq has been told by Harmonix the tracks will release later this year.
Also, while they're on the subject, transferring the original Rock Band tracks to the sequel is really as simple as purchasing the license. No muss, no extra fuss.