I personally believe that Guitar Hero II was an example of epic fail when it comes to downloadable content. They promised the world and brought us mostly mediocre songs from Guitar Hero and a few extras near the end of its life. Rock Band doesn't plan on making that same mistake and other then latter addition of the full CD downloads, below is a list of some of the early songs they plan on adding.

Police pack: Roxanne, Synchronicity II, Can't Stand Losing You (all masters)

Queens of the Stone Age pack: 3's & 7's, Sick Sick Sick, Little Sister (all masters)

Metallica pack: A three or six song pack, no songs announced yet

Black Sabbath pack: N.I.B, Sweet Leaf, War Pigs (all covers)

David Bowie: Moonage Daydream, Heroes, Queen Bitch

Punk Pack: Ever Fallen in Love, by Buzzcocks (cover)
I Fought the Law, by The Clash
Rockaway Beach, by Ramones

Single Songs:

Fortunate Son - Credence Clearwater Revival
Jukebox Hero - Foreigner
My Sharona - The Knack
Cherry Bomb - The Runaways
Bang a Gong (Get it On) - T. Rex
Joker and the Thief - Wolfmother
Brass in Pocket - Pretenders
My Iron Lung - Radiohead
Buddy Holly - Weezer

Source 

 

Buddy Holly, Sabbath, and The Police? Sign me the heck up. I thought my wallet would burn like a rash from the bundle alone, now I know it will continue into the forseeable future.