Kotaku describes how to access this easter egg:
It's very easy to do. You have to [have finished the game] on normal or hard, or on New Game+ (Which you can't unlock by completing the game on easy, anyway.) At any point in the game, take out Batman's Cryptographic Sequencer. While holding down both RT and LT (or, I presume, R2 and L2 on PS3), begin to rotate your left thumbstick clockwise and your right thumbstick counter-clockwise. After doing that for a few seconds, you'll notice that the Dark Knight's chin has gotten a good deal…chinnier.
You can also watch the Rooster Teeth video (great channel for achievements and easter eggs, by they way!):
On the PC, it's a bit different. You don't seem to need the cryptographic sequencer. The key combination is:
or in WASD:
And here's a video: