mrstino.... that's a toughie. If I say anything.. I say it in a casual, joking manner. I then make sure that I say one thing and one thing only. Don't press the issue any further unless that person seems receptive and asks questions.
One problem that I have that goes hand in hand with that is when, in a large group of people, most of us pay in cash for our bills and then one person wants to pick it up on a credit card. I always tip 20-25% (previous pizza delivery chick) and have seen my credit card weilding friends leave a 10-15% tip on their card. So I know that they're pocketing everyone else's generous tips and it irks me to no end!