To get a 1-day pass for the US Airways Club (which I think is $35 for 1 visit or $50 for multiple visits to multiple clubs in 1 day), just call the Pittsburgh club you want to go to; contact information is at http://www.usairways.com/pro_services/index.htm

I have visited the Club in the center of the airport and it's fine, although a bit dated appearancewise. There's another one there as well; I don't know anything about it.

You can pay by credit card, they'll mail you a receipt and they can list your name in the guest book at the Club for the day you want to come; no need to worry about getting a paper certificate or anything.