There was recently a news article in the Daily Mail about two gentlemen who were declined boarding for First Class due to a dress code. I like to dress in business attire because my slacks are often more comfortable than my jeans and I feel like I am taken more seriously when I do, but I also know that often the worst dressed people in First are often some of the highest paid people there (actors, musicians, other celebrities trying to lay low by dressing down or .com execs who shun business norms) and I never judge anyone in First by how they dress.
That being said, US Airways and other airlines do have a dress code for employee travel benefit users.
First or Envoy Class: Pass travelers may wear casual attire, including blue or black denim attire, skirts, capri-style pants, and sandals, provided it is well groomed, neat, clean, and conservative. Unacceptable attire in First Class/Envoy includes tee shirts, shorts, jogging suits, athletic gear, baseball caps, athletic shoes, beach footwear, flip flops including Croc style footwear.