It wasn't just any court, but the U.S. Tax Court, and the IRS was the Respondent. The tip issue was a minor part of the opinion, which cannot be used as a precedent, but it shows which way such a case will probably come out in the future. They are saying that tips are indeed a legitimate expense, but you have already been reimbursed via your per diem allowance.

http://www.watsoncpagroup.com/images...tinentalFA.pdf