Apple’s iPhone has finally secured a foothold, albeit it a tiny one, in China – another step towards opening up the world’s largest mobile market to the global ecosystem developing apps and web services for 3G smartphones. China Unicom, which now has more than one million users on its 3G W-CDMA network, told Reuters today that it has sold 5,000 iPhones since the iconic handset was launched last week.

In a country of more than 1.3 billion people, that doesn’t amount to a blockbuster opening week for the iPhone. But we shouldn’t get too distracted by the fortunes of one handset. The big picture is looking good for apps developers and marketers wanting to use the smartphone channel to target the vast Chinese market. Now that it has a W-CDMA network, China Unicom can offer its customers scores of tried-and-tested 3G smartphone models from the likes of Nokia, RIM, Samsung, Sony Ericsson, HTC and many others. China Unicom is aiming to add one million new 3G customers every month, according to Reuters.

At the same time, China Mobile, the country’s largest operator by far, is pushing manufacturers to develop smartphones using Android and other open operating systems, as it tries to attract customers to its new 3G TD-SCDMA network. China Telecom, deploying a CDMA 2000 3G network, is also likely to turn to smartphones to lure affluent customers away from its rivals.

As a fancy handset is increasingly a status symbol in a country where only a small minority can afford cars, China could easily have 50 million smartphones in circulation within two years.

Although western developers shouldn’t underestimate the cultural differences, the Chinese are likely to have a big appetite for mobile apps. Even on sluggish 2G networks, they are vociferous users of mobile data services, downloading music and sending each other picture messages much more regularly than their counterparts elsewhere.

Moreover, the intensifying competition between the big three mobile operators, following the government-led restructuring of the industry, should help developers to find routes to market.