Prepaid SIM Card

I'll be visiting China in April 2013 and will be staying for an extended period. I'm told to get a pre-paid sim card for my mobile phone as this will be cheaper. However, a friend of mine told me that this is not possible as foreigners were not allowed to purchase these pre-paid sim cards. She tried buying them last year when she was in Sichuan and got turned down. Please advise.