In China, only two airports have SIM card shops - Guangzhou airport & Shanghai Pudong. You don't need to show your passport. Just simply pay and get it.
Foreigners cannot buy at other places and shops as they need a local Identity card to register. Alternatively, you may buy it in black market or ask a local friend to do the registration with his IC.
Buying a SIM card in China is troublesome for a foreigner as they don't accept passport.
They do now... I got mine at the Unicomm shop in GZ city area. Used my passport for registration.