Prepaid cards are still easily bought but can be activated only after you've completed real-name registration with your passport, normally at a customer service centre, not one of those small shops.
One of my friends told me that the last time he went back, he had to go to a company customer service center, and he was asked to record a statement saying that he wouldn't use his phone for things that the government didn't want you to do in addition to the passport thing. I thought that was kind of weird.