T-Mobile USA is testing an HSPA+ network on its 1900 MHz PCS spectrum at San Francisco’s Moscone Center, where Apple will hold its World Wide Developer Conference (WWDC) next week.

In a statement, T-Mobile said the test network was deployed as part of a routine network upgrade inside Moscone Cente,r and timing ahead of Apple’s event was purely coincidental. The network would allow attendees with an unlocked AT&T iPhone to use T-Mobile’s test network with one of the carrier’s prepaid data plans.

T-Mobile reports that it has more than 1 million iPhones on its network already. As part of its $4 billion network modernization initiative, the company plans to re-farm its 1900 MHz PCS spectrum, currently devoted to 2G customers, for use in deploying an HSPA+ network that would welcome a more diverse group of unlocked devices onto its network.

The company has said it hopes to have that network rolled out in a number of markets by the end of the year.

Without actually offering the iPhone as part of its portfolio, the question of the day for T-Mobile has been how to attract new customers without Apple’s iconic device.

During a first-quarter earnings call, CEO Philipp Humm stressed that 45 percent of T-Mobile’s gross customer additions were unsubsidized, SIM-only customers who bring their own devices to the network.

To add some perspective to how bleak T-Mobile USA’s fourth-quarter 2011 customer losses were, consider that the loss of 510,000 branded contract customers in the first quarter was in fact a 28 percent improvement sequentially.

Apple’s WWDC runs from June 11-15 at San Francisco’s Moscone Center. Google will hold its IO developers conference at the same location less than two weeks later, from June 27-29.