If you want to get them as soon as possible, you can sign up now. Head to the official site, and you can enter the code on the back of your Rock Band 2 manual. You'll have to fill out a survey and enter your email address -- come on, nothing is free -- and then click a confirmation link in the email they send you.
Once you've done that, all you need to do is sit back and wait for an email with a code that'll let you grab the tracks. While only Xbox 360 owners can do this right now, PlayStation 3 and Wii owners will also be able to enter their codes just as soon as their versions are on store shelves later this year.
Wait: Wii? You mean that Rock Band 2 for Wii will have downloadable tracks? Well, I'll be.
Sign Up For Your Free Rock Band 2 Tracks | Game | Life from Wired.com
