http://www.dol.gov/elaws/faq/esa/flsa/002.htm
Now that we have the definitive word on the law, I personaly think it's lame not to tip the people serving you. The guy helping you in the department store is not putting up with your complaints, whining, screaming kids for an hour or more as a rule. They do not have to watch for spilled food or drink on the floor or clean up after some really nasty people. They don't have children sticking food into their pockets or be verbally abused because someone else didn't make the plate to their exacting standards.
I always tip the server but not an exact amoumt, it depends on the quality of the service. I tip the person who delivered pizza(back when we still ordered), I've tipped other delivery people as well. I do not tip for carryout.
You don't tip the guy(or girl) who prepared your food because generally they make better money unless they work fast food or low end place. You can get anyone to carry a plate(no offense to those who do) without too many issues but your business depends on your cook or chef. You don't just grab anyone off the street and you pay accordingly.
Now I'm going to pull an Eddie and complain about too much time devoted to a topic that already has it's own thread.