Well it depends on when you are going.. if it is in 2007 the gratuity is included at 18%.. if it is in 2008 than the gratuity is not included and you will have to decide how much tip to leave
In 2008 all parties of 6 or more will have an automatic 18% gratuity added. You can then pay this bill in cash, credit/debit card or room charge (probably also disney visa rewards card and disney gift card).