T-shirts
Enter one of the following at the start screen:

Unlock GamePro t-shirt: DOWN, R3, UP, R3, L3, DOWN, L3, UP, DOWN, SQUARE
Unlock GameStar t-shirt: UP, R3, RIGHT, R3, R3, LEFTt, R3, DOWN, CIRCLE, CIRCLE
Unlock Harmonix t-shirt (Ishani): L3, CIRCLE, UP, CIRCLE, SQUARE, L3, DOWN, DOWN, R3.
Konami T-shirt (Dwayne): RIGHT, R3, RIGHT, R3, SQUARE, RIGHT, CIRCLE, SQUARE, DOWN, LEFT.
Game Informer t-shirt (Angela): DOWN, L3, L3, R3, R3, CIRCLE, RIGHT, SQUARE, L3, R3

Unlockable List
Beat the game with any character on single play showtime mode and get all Platinum records. You'll get:

Hidden characters:
The Dude
Helga
Sorrow
Vox
Joe

Alternate Costumes for:
Ishani (Business outfit)
Ishani(Retro silver foil outfit)
Dwayne (Mullet outfit)
Dwayne (Purple and green superhero outfit)

Unlockable Venues:
The Garage
The Wagon Wheel
HMX Records

Unlockable Songs:
Rich Girl
I've Got You Under My Skin
Virtual Insanity

Unlockable Video Extras:
Wavegroup sings
Harmonix Sings
Playtesters Sings
Konami Sings
Wavegroup