Check The Full Rules Here: http://www.rockband.com/news/pepsi
I'll break it down:
Buy any Pepsi products, starting today apparently, that have a screw-on Lid (no Cans) and you can get free Rock Band DLC tracks. Not a bad deal for getting something to drink and some songs. Let's just hope they're not only giving away certain songs, like i-Carly full albums or (God Forbid) Guns N' Roses' "Shackler's Revenge." I will investigate later today and get back to you peeps.