the only operator that will sell SIM cards and allow use of phones you bring from outside of China is UNICOM
China Telecom and China Mobile's "3G Service" require their own handsets as they use a different frequency set than Operators outside of China
Unfortunately, NONE OF THESE SO CALLED 3G services are even close to Edge or EVDO speed in the real world.
I am using Unicom now and its horribly slow and problematic and very expensive comparably.
I used 2G for 3 years with China Mobile and see no significant difference other than a huge difference in price.
When I do roam in HK on Unicom's 3G it is as fast as US speeds.
Just got my China Unicom service suspended cause I apparently spent 300 yuan worth of 3G access in less than 15Days
However the 3G Service is pretty good