Using my AT&T in China

Yes, but if your modem firmware is higher than 01.59.00 then it cant be software unlocked.
You can get a gevey sim to unlock and use a local microsim.

 

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.

 

I'll be in China in a couple of months. Should I purchase the Gevey before I leave or are they readily available for purchase over there?

 

purchase it at home just in case, im using a iphone 4 software 4.2.1 firmware 3.10.01 with a gevey sim in taiwan and it works perfectly.