If you have a GSM phone, this is a great resource for travelers:
http://www.gsmworld.com/roaming/gsminfo/cou_cn.shtml
Notice that there are two mobile operators (China Mobile / China Unicom). They both run on GSM 900. For most Americans, this means you will have to have a "quad-band" GSM phone. Many of the newer models these days are quad-band, but you should check on your specific phone model.
This will allow you to roam on both of these networks paying whatever your cell carrier charges for roaming in China. Now, you may want to look at getting a local SIM card to use in your phone once you arrive as that may make dialing/receiving considerably cheaper.