T-Mobile USA posted another disappointing quarter, losing 318,000 lucrative postpaid customers as its profit fell 12.4 percent to $268 million. As ZDNet notes this morning, the bleeding stands in contrast to the rest of the nation’s tier-one operators, each of which has been adding subscribers. And it demonstrates that T-Mo has failed to capitalize on its move to roll out the speedy HSPA+ technology that it controversially markets as “4G.” So why can’t T-Mo turn the ship around? I think it’s the lack of a high-profile, sexy device backed by a big-budget marketing campaign. Unless T-Mo is going to get the iPhone soon — which doesn’t seem likely — it would do well to pick a single great handset (or small line of devices) and advertise it as much as possible.