T-Mobile marketing chief Cole Brodman claimed that as of September 2011 there were over one million unlocked iPhone users on their network, and that T-Mobile offers a "no-compromise" iPhone experience. And even though T-Mobile has shown great interest in carrying the iPhone, they are still waiting on Apple to make that a reality.
In the meantime, however, T-Mobile will start providing official support for the iPhone starting January 30th, 2012. The carrier will provide technical support, information about the iPhone's features and specifications, and answer basic questions about the device. So even though T-Mobile is the largest carrier in the US that doesn't have the iPhone yet, it'll be more than ready to facilitate it when Apple decides to launch it on their network.