Officially you'll need ID, in reality if you don't buy from a carrier-owned or other official store you won't necessarily be asked. In either case, a passport should suffice, although depending on how much experience they have dealing with foreigners it may take quite some time for them to find a way to process it all.

The only W-CDMA provider in the PRC that can sell you a 3G SIM for your iPhone is China Unicom 中国联通; China Mobile uses a proprietary, China-only 3G standard (S-TD-SCDMA) that falls back to standard GSM/EDGE on non-compatible hardware, while China Telecom uses CDMA2000 1x EV-DO that wouldn't work with an iPhone at all.