the pilots of USair are too lazy to turn the light off. if you go to the usair site, you will read of sign on on full transatlantic. many times the sign has stayed on all the way on my dca-dfw flight. i had a blood clot in my leg last year. when i get on the us air flight, i tell the FA's, and tell them i am going to get up and walk every 30 min.
i think getting up when the sign is on is a violation of some govt reg, and you may end up chained to your seat until the fbi removes you at the end of your flight, unless they decide to land in gandor to remove you.