In theory a China Mobile SIM card should be able to provide 3G services. In practice I have not been able to get my (3G only) HTC phone to access the internet via such a card. China Unicomm seems to be a better offering in the 3G field. While China Mobile supposedly provides both 2G and 3G services, in practice it is a 2G service.
Buying a SIM may be best done at a big telecomms store, rather than the airport - you are more likely to get a solution that actually works. A purchased SIM does seem to be the way to go. Be aware that if you do buy any SIM in China you will be subject to the constraints of the Chinese Firewall, so although you can access data, you will not be able to access Facebook, Blogger, or (sometimes) GMail, and other services.