I believe that fashion is fashion for the sake of fashion and doesn't always have or need a meaning. While it's true that a cross is a talisman for many Christians, it also holds other meanings to different subcultures and faiths. The cross shape dates back to before the Christian religion, has been a part of Paganism for centuries, and has been adopted (maybe subversively) as part of the Gothic culture.
Just because your talisman or symbol becomes a trend doesn't make it any less meaningless in your eyes, and that's what counts, right? I symbol has the power other people give it but it is most powerful with the meaning that the wearer gives it. If you want to wear a cross as a fashion statement, don't let being PC stop you.
Pics via Blue is in Fashion, Trendy Crew, ZeroChan.net, HNaotoUSA.com, 80's Fashion, Midi-Nette and Poupee Girl.