An article in the Journal of Accountancy http://www.journalofaccountancy.com/News/20125925.htm summarizes the new IRS guidance on tip accounting in Rev. Rul. 2012-18. In a nutshell, if the amount is not at the patron's discretion (e.g., "A gratuity of 18% will be added for all parties of 6 or more,") then it's a "service charge," not a tip. When paid out to employees, it's categorized as wages, not tips.
Simple, right?
Stay tuned - guidance should be forthcoming on how many angels can dance on the head of a pin....