I've never been charged tip on any of my stays and until recently I always included approx 15 percent additional tip. However, after reading this thread I now put "Diamond/Included" on the tip line.

At some locations, notably PH Saigon, they ask for your room number up front, they seem to keep a list of Diamonds and once they cross reference your room # they know your breakfast is complimentary. No bill, no signature, just order off the menu, eat, and leave when you're finished. I wish all Hyatts did it this way.