According to an article on the Public Knowledge website, the US Supreme Court is currently debating a case regarding fair use of clothing and costume design when such designs are otherwise protected by copyright. Right now, the case involves cheerleading uniforms, but it's not that far off from costumes.
The argument is that copyright protects "creative works" vs utilitarian items---the example given being that a statue would be protected, but not something like a chisel that made it. However, many items fall into a gray area that may be both functional and creative. For props, consider a leg lamp from the movie A Christmas Story: both creative and functional. For costumes, well...is it serving as a costume to transform you into another character for a particular purpose, or are you simply stealing someone else's creative look for your own benefit?
What about those who make and sell replica props and costumes?
Let us know what you think in the comments...we'll be watching this one closely.