Cruisermeg is correct. You can get cash from the casino, but you pay a 3% fee for doing it.
On our last cruise (2-1-09, Explorer), we had over $700 in on board credits. We used some of them for excursions, but took the majority in cash from the casino. I would rather pay the 3% fee than lose the credits. We used the cash to pay our gratuities, and put the rest in our pockets. I'll never leave unused credits on my account.