Much like T-Mobile in the US, China Mobile's 3G technology isn't compatible with Apple's iPhone, so unofficial iPhones are a strictly 2G only affair. While there are over 1 million unofficial iPhone on T-Mobile users, that's dwarfed by China Mobile's 15 million unofficial iPhone users. An unofficial number big enough, it seems, to garner a little official support from Apple. According to The People's Daily Online:
China Mobile said that Apple had updated the system of all iPhone 4S handsets manufactured after Feb. 8, 2012 to fully support its network, and will offer a software update for previously manufactured iPhone 4S handsets in early March.
The issue apparently stems from the iPhone 4S supporting the P frequency band (885-915MHZ) but not E frequency band (880-890MHz). China Mobile supports both, primarily E for indoor and P for outdoor. So if a user tried to connect while on the E band, the iPhone 4S wouldn't work.
China Mobile has 665 million subscribers, so even though it's not an official iPhone carrier today, it's a huge potential market for Apple. It'll be nice for everyone when the iPhone 4S unofficially works better on China Mobile. It will be something else entirely when a future iPhone goes official.