As everyone else has said, you'll be far better off either bringing an unlocked smartphone with you and picking up a local SIM or just buying an inexpensive one once you're here.

A few points worth noting:

** If you want 3G on a non-Chinese phone, Unicom is your only option (Mobile uses a homegrown 3G standard).
** Likewise, if you buy a phone here and plan on using it elsewhere, make sure you don't pick up a Mobile-specific TD-SCDMA model.
** Whatever you do, don't buy an iPhone in China; they're horrendously overpriced, which is great for those of us making a side business out of importing the things, but not so good for anyone else.
** Check Taobao for prices before heading off to the local electronics market. Plug in a brand name or model in English and see what comes up; add about 10% to the prices and this should give you a rough estimate of a fair brick-and-mortar price. Actually, this is a good strategy when shopping for just about anything.
** Contrary to what you might think, most of the homegrown Chinese brands will actually work fine, but might not have good English interfaces or, as bridgeair mentions, Google apps (but this is easy enough to fix).