My SIL went cle-phl-(msy) on US. She took a bit of a large "carry-on" and they tried to make her check it. When she refused, the the agent was like, "I know, you're getting off at philly" and tried to make problems for her. She was scared, but there isn't mush they can do. The agent ran to the gate to try and make sure that they don't let her bag on, but it was already gate checked.
On the other hand I've booked for many people, and no one else has had a problem.
In short, just don't put down your FF#, and don't check luggage, and like Eli said, what can they do already?