Harmonix and MTV Games have set live the registration page for the 20 post-release downloadable tracks that come free with your purchase of Rock Band 2.
You simply have to enter the code found on the back of your game's instruction manual and you'll be all set to go. Unfortunately, the code you must redeem to get the 20 free tracks isn't being sent out just yet; we're still in the dark as to what these songs will be, and how many of them also appear in Guitar Hero World Tour.
Of interesting note to Wii owners, the registration page states that Xbox 360, PlayStation 3, and Wii are all elligible for the 20 free tracks. Unless Harmonix and MTV Games plan on sending out discs, that likely points to one thing -- downloadable content for the Wii version of Rock Band 2 confirmed. Maybe.
Now about that storage issue ...