I bought Rock Band 2 before Christmas, to be played casually when friends come over. So I wanted to unlock all the songs so we would have the greatest variety to play, but I found a variety of different codes to use for the PS3 version. I couldn't get the first code to work, tried it multiple times. Then after more googling, I found the second code, and it worked fine.
*Note: this code must be entered using the drums*
The code that worked:
Main Menu > Extras > Modify Game > Red, Yellow, Blue, Red, Red, Blue, Blue, Red, Yellow, Blue > scroll down and select unlock all songs
Apparently I will need to select this option each time I start the game, but I shouldn't need to enter the code again.