Reportedly, This Tuesday Harmonix will be releasing 9 tracks, mostly from Rock Band 2, as free DLC for Rock Band 3.

You can download it through the usual DLC pathway on the Xbox 360 or PS3, titled “Rock Band Free Pack 01" starting this Tuesday 1/4/2011.

Eight-Pack of Johnny Cash Classics Come to Rock Band! // Blog // Rock Band®

Rock Band Free Pack 01

Abnormality – “Visions”
Anarchy Club – “Get Clean”
Bang Camaro – “Night Lies”
Breaking Wheel – “Shoulder to the Plow”
Libyans – “Welcome to the Neighborhood”
The Main Drag – “A Jagged Gorgeous Winter”
Speck – “Conventional Lover”
The Sterns – “Supreme Girl”
That Handsome Devil – “Rob the Prez-O-Dent”

And do yourself a favor - delete Anormality - "Visions" before you even try it. It's horrible.
Bang Camaro - "Night Lies" is a ton of fun.