Heading to PEK soon and was wondering about the best way of renting / obtaining a mobile phone there as the one I have here won't do. I only need it for local calls / general getting around the city...

What makes you think it won't work? Beijing has both GSM and CDMA systems in place. Verizon phones work just fine in Beijing, as well as many other places in China (not sure about Sprint). Cingular/AT&T phones will work there as well.

Instead of renting or buying a mobile phone, just get yourself an unlocked tri- or quad-band GSM phone (you can get them on eBay for as little as $30), and pick up a SIM card once you get there. If you're not concerned about cost, you can pick up an over-priced (but still not particularly expensive) SIM card from one of the vendors in the hall where you retrieve your luggage. Otherwise, there are stores on just about every corner that sell reasonably priced SIM cards -- basically any corner market that sells cigarettes and bottled water will also sell SIM cards. In China, SIM cards are priced according to the phone number that you get. The Chinese are superstitious about numbers and will pay more, sometimes a lot more, to get a "good" one, e.g. one with lots of 8s and no 4s. If you're not picky about your number, you'll find SIM cards that are dirt cheap.

We have a couple of GSM phones we got on eBay that we keep just for international travel. GSM is standard virtually everywhere in the world, and pay-as-you-go SIM cards are available with minimum fuss and at low cost everywhere.

....as does Sprint, but roaming on all of these guys costs big bucks, both for you and the people in BJ who might want to call you (actually, the current rates are reasonable, but local calls are still preferable, especially for those that might not have your spending power).

If you travel to China often, I recommend getting a phone in country because it will support Chinese characters (though many newer phone models can be upgraded via downloadable software fairly easily). I gave the same advice to my "Huairou cousin" in advance of his arrival here; however, he ignored me on the grounds that he didn't speak Chinese at the time (his subsequent progress has amazed me, but that's another topic). In any case, he now finds himself helpless whenever he receives addresses in Chinese or SMS's from his new friends.

Furthermore, I should point out that all SIMs are not created equal. For example, Unicom offers free incoming calls on its prepaid numbers (both GSM and CDMA). Meanwhile, China Mobile numbers are more reliable, but also more expensive.

In several previous threads, I've advocated the post-paid plans because they are MUCH cheaper in the long run, but I'll refrain from discussing those in this thread.