Update:  Good sign - we’ve heard this announcement when the March DLC was leaked:

Sean Baptiste over at Harmonix has given an official statement on the leak we announced in our original post (below):

“This list of Rock Band DLC that was discovered within the files of a recent DLC pack is unconfirmed and subject to change. Please stay tuned for confirmation of Rock Band DLC closer to their release.”

Update 2 [Mike]:  Found a better version of the list from shacknews.com and posted it below.

After hacking into some .bin files contained within the latest Rock Band DLC pack, haxxor laconic at the ScoreHero forums has supposedly discovered what looks to be a “complete” list of Rock Band content. While the alleged presence of these tracks is no guarantee that they’ll appear in any future updates, seeing such an exhaustive list of songs from artists who are already featured in the game (Nirvana, The Police, Smashing Pumpkins) doesn’t look like a coincidence, either. The complete list of songs - which obviously includes the stuff already available - can be found below. Note the stacks of unused Nirvana content , more NIN, cool Skynyrd etc… Keep your fingers crossed and wait for the beep.

 These songs have since been released (helping to confirm the rest of the list) 

Blur:
Bettlebum

Foo Fighters:
D.O.A.

Garbage:
Why Do You Love Me?  

Grateful Dead:
Franklin’s Tower
Casey Jones
China Cat Sunflower

I Need a Miracle
Sugar Magnolia
Truckin’

Iron Maiden:
Wrathchild

Jonathan Coulton:
Still Alive

Muse (or Def Leppard):
Hysteria

Nirvana:
Breed
Comee As You Are
Lithium
Lounge Act
On a Plain
Polly
Smells Like Teen Spirit
Stay Away
Territorial Pissings
Something in the Way
 

Oasis:
Rock n’ Roll Star

The Rolling Stones:
Brown Sugar
(I Can’t Get No) Satisfaction
 

The Smashing Pumpkins:
Zero

Stone Temple Pilots:
Sex Type Thing

Tokio Hotel:
Through the Monsoon

Weezer:
El Scorcho

The Who:
Behind Blue Eyes
Pinball Wizard

Not that this should need repeating, but just because this list of songs were found in a game file doesn’t mean that they’ll definitely be constituting future DLC. Song list found in DLC files

Sources: Scorehero, Joystiq, shacknews