Even though iPads aren't officially available in China yet, there's no problem using them here, even the 3G versions, since (unlike US iPhones), the iPads are not SIM-locked to AT&T.
You first need to get a China Unicom 3G SIM with a data-only plan. You cannot use the 3G from China Telecom or China Mobile because they use different (read: incompatible) 3G standards. There's more information here on what to get.
Next, since the iPad only takes the new microsim size cards, you need to cut the China Unicom SIM card to the right size. This is pretty simple. I used the template on this site and had no real issues. Just remember to measure twice and cut once.
Once you insert the SIM into the iPad, go to Settings and turn on Cellular Data. Your iPad should find China Unicom right away. You need to set the APN settings; "3GNET" is the APN, username, and password.
Tada! You should now be surfing at a pretty fast speed, anywhere there's coverage!