Chinese Mobile Phone OperatorsEdit
There are three major mobile phone operators in China. China Unicom, China Mobile and China Telecom. They use different radio frequencies that require a local type of mobile phones. China Unicom is the one operator using the same system that is common in Europe and most other parts of the world.
Ie - even if you go to a China Telecom office and buy a sim card with 3G data, the speed your EU/US phone will work on will only be 2G.
China Unicom is officially the worlds third largest mobile phone operator (current info?). China Unicom is also the official iPhone retailer in China. The company operates a GSM and 3G based network covering most of China. The 3G network using WCDMA technology was launched in october 2009.
Subscription startup costEdit
Deposit of RMB200 required - but monthly charges can be taken from this.
To recover deposit - cancel contract at originating Unicom office, wait 2 months, collect deposit. Overseas visitors requiring Chinese PAYG sim cards, you will first need to show passport.
Data feature packsEdit
A range of rates from RMB96 per month including 300MB to RMB886 for 3GB. These require subscriptions. 
However, a separate payment structure exists for data-only devices (like the iPad). 500MB nationwide for 50RMB, 1GB for 80RMB, 3GB for 150, 5GB for 200, and 10GB for 300.
In addition, there are one-time prepaid data cards available - 3GB for 300RMB and 6GB for 600RMB. These are good for half a year and one year from date of first use respectively. These are probably your best choice as a tourist or business visitor.
Asking for 'data only sim-cards' may confuse the Unicom sales people. Instead ask for 'for iPad' or 'for internet'. You need to provide your passport and a phone number (ideally for your hotel) and the process is more time consuming than expected. The clerk that does the actual data input speaks no english what so ever and is tucked away in a side room.
It may be faster to purchase the "China Unicom Ipad Sim" from a generic mobile shop. They did not require a passport - the whole process took about 1 minute.
Is it working?Edit
As soon as you put in the SIM card and restarts the phone you should recieve two text messages from number 10010. The first one is a greeter and the other one tells you how much data you have left to use. Everytime you loose connection with the network (quite often unfortunately) or turn your phone back on you will get these two text messages. They pile up.
Regular size pre-paid SIM cards should be available from actual Unicom offices - high streets in China will have tens of outlets, look for the big one.
Sign up as per PDF from this site 
The form provided at the above web site does not mention this, but when I went to buy my SIM card, they wanted the address where I was staying. Best thing to do if you are in a hotel is bring one of the address cards that they give you in the hotel, and give it to the person who is entering your information.
Very few employees at a typical China Unicom office will speak English. They are expecting to serve Chinese-speaking customers, and if you don't have any Chinese you will need special help. When I was there they bent over backwards to help me, but I realized afterwards that it was a major imposition. So the more prep work you can do up front, the better—don't walk in blind and assume it'll be okay. The english-speaking person who helped me at the office did not do the data entry, and did not know what information would be required; the data entry person didn't speak english.
Alternatively, one can avoid this hassle by finding the nearest computer parts or cellphone market and buying a card there. No registration required- cash paid and that's it. But you may need to haggle the price down to reasonable levels.
Both regular sized and microSIM cards very readily available at the major Unicom outlets in Beijing.
Requires further deposit - this varies from conversation to conversation but RMB300 seems common. Use a callback service on iPhone.