What could be causing my weak wifi signal on my iPhone 3GS?

All of a sudden my wifi signal range is very short on my 3GS. My wife has the same model and her range appears to be just fine. I am also having problems in some underground 3G service areas like the Washington, DC subway system. Certain underground stations carry the 3G signal but all of a sudden that is non-existant like my short range on my wifi. Any advice would be appreciated. Thank you.