Getting Introverts to Unwind at Work X-Mas Party?

pongo asks: "My department has a large population of introverts, as confirmed by Meyers-Briggs testing. Somehow I was suckered into planning a holiday party with another department that suffers similiar symptoms. Any suggestions on ways (themes, decorations, food choices, games, etc) to encourage light banter and to get people meeting each other, which would make this party a big success? The party will be during business hours and alcohol is allowed." The key here is making a comfortable atmosphere where everyone can unwind and feel comfortable with meeting each other. What ways do you know of to encourage this kind of atmosphere?