Nintendo Power had a short article about this, and listed some of the confirmed songs:
Wolfmother - Dimension
Coldplay - Violet Hill
Lenny Kravitz - Where are we Runnin'
Evanescence - Sweet Sacrifice
The Strokes - Reptilia
The Bravery - Unconditional
Fall Out Boy - This Ain't a Scene, it's an Arms Race
Wheezer - Everybody Get Dangerous
The Kaiser Chiefs - Ruby
The 12 Stones - Adrenaline
Tenacious D - The Metal
Atreyu - Falling Down
I don't see why they can't make these games without including a handful of songs from previous Guitar Hero games. It didn't bother me at first, but you'd think "Modern Hits" would limit itself to songs that came out after Guitar Hero 3. Aside from the somewhat intriguing idea of playing an Evanescence song in a GH game, I'm not too really impressed.