After rescuing Cash and an orderly from the Joker's laughing gas, face the room where they were previously trapped while in the main room and you will see a ticker scroll the following message:

"Dr. Clooney report to Gynecology." Having previously starred as Batman (Batman & Robin - 1997), this most probably hints towards George Clooney.

You can unlock Armored Batman (more health than regular Batman) for the Challenge Mode once you have successfully beat the Story Mode. You can then choose to play as Armored Batman before you begin a challenge.