(This is an official "Jayne Cobb" hat, available via ThinkGeek. Attractive, no?)
That's because hats are "useful articles" under US Copyright Law - and so can't be protected by copyright. This peculiarity of copyright law has bedeviled fashion designers for years, and while there has been some chipping away of the concept (costumes, for example, can be protected), in general an article of clothing can't be.
So if you've received a cease-and-desist letter because you are selling "Jayne Cobb" hats, give me a call. I'm a big "Firefly" fan and will be happy to review your situation and give you some idea of where you stand. Browncoats do need to stick together, after all. Especially when the Alliance comes a calling.
(Update: The folks at ThinkGeek have decided to donate profits from the sales of officially-licensed Jayne Cobb hats to the Firefly-inspired "Can't Stop the Serenity" charity. Well-played, Geeks, well-played!)