Got a 3G card for China Unicom, popped in my unlocked iPhone from the States, and it works fine. No set-up or registration required, paid 180 RMB for the card and 2 months of service (96/mo more or less).
Speeds are about the same as AT&T back home.
This is by far the easiest route: no set-up, no problems, no figuring out what works with what...
iPhone + China Unicom 3G card + 100 RMB per month in prepaid service = fast and easy internet on a great phone.
Had all kinds of HTC, Dopod, HP, etc. and I finally got tired of all the settings and compatibility problems.
iPhone works like a charm, Google maps is super useful, as is being able to check my e-mail and sign in to Skype no matter where I am.