Gah! St00pid airlines.
OK, so I get to check-in at Gatwick, and find that US Air have changed my connecting flight from Philadelphia to Chicago for one an hour and a half later. And no, there are no other available connecting flights. So nice of them to contact me in advance to warn me that they will be screwing up my plans. It's lucky that the girl at check-in just happened to notice the change, or I could easily have gone on in blissful ignorance :(