Concerns over the cost of the iPhone and lack of certain functionality (such as Wi-Fi) were cited as potential reasons. China's largest phone company, China Mobile, relies heavily on people who pre-pay for their phones -- a situation that doesn't mesh well with the iPhone, which requires a constant Internet connection. Inclusion of Wi-Fi down the road might help Unicom out.
Current sales may be low, but prospects and hopes are still high. Both Apple and Unicom expect the current situation to change. Some analysts feel that over the course of the next year, China will account for a massive 15 to 20% of all iPhone shipments, totaling around seven million devices. They'd better hope their first week is a fluke, or I don't see that happening.