I am using AT&T iPhone 4 in China. I unlocked it with Gevey SIM.
China Unicom has micro-SIM, they are the iPhone official carrier in China, you get 3G with them.
CMCC (China Mobile) is bigger and more popular, they use regular SIM, but at their store they will cut their SIM into micro-SIM for you, free of charge. They have cheaper pre-paid rates, but no 3G.
If you use alot of data, I suggest you go with China Unicom.
I am stuck with CMCC as I had the phone# for years and don't want to change, in China you can not port numbers between carriers.