It's been harder for non-residents to buy prepaid Sim card in mainland China and registrations with the "authorities" being mandatory with ID proof, as least officially. It sounded like the People/HK China Mobile dual # might work quite nicely for you, as I use it both in HK and Shanghai myself + Vietnam (rates being most expensive in VN regardless of carrier) - the prepaid Sim card has the daily/weekly & month unlimited data package as add-on, very cheap & a fraction of what the high end hotels would charge as enhanced services (when not included in your bundled fees).

Take an unlocked 3G GSM phone in the proper Asia bands to take full advantage of speed - there are also free wifi (open) public hotspots, accessible to most smartphone users. The CMHK local store branch in Kowloon on Nathan Road, like others, have extended hours into the evenings & very competent in getting my unlocked Verizonwireless global HTC phone the APN & proxy settings to surf on their network, and tethering is allowed.

I kept the CMHK dual # sim active, refilling it once every 180 days only cost less than $7 USD ($50 HKD as mininum online refill - which I do directly on their website here in the States, no need to go thru buying PINS & all that) - it roam here on AT&T and T-Mo's native networks, so I never miss a call or VM when my HK/CN number (distinctive ring tones can be set) are called. Frequent HK/CN travelers should find this very useful as I can leave my "local" contact numbers before I leave, use it for my hotel bookings & return flight reconfirmation, etc. and as soon as the bird lands on HKIA & phone is switched back on, I'm "live" again & calling. Plus, the added Google Voice & Skype setup is making it very manageable and seamless as I travel around ....