Is there an unlock all cheat for rock band volume two?

Yes, here are the cheat codes to unlock almost everything in the game:

• To unlock all venues, input the following code while at a venue:

Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start

• To unlock all songs, input the following code during gameplay:

Down, Up, Up, Down, Right, Left, Left, Right, A, B, Start

• To unlock all band members, input the following code at the character selection screen:

Down, Up, Down, Up, Left, Right, Left, Right, Y, X, Start

• To unlock all difficulty levels, input the following code from the main menu:

Up, Up, Up, Down, Left, Right, Left, Right, A, B, Start