Boost Mobile, Sprint’s prepaid service, was the struggling carrier’s surprise winner in the first quarter, after it racked up 764,000 new pre-paid customers eight weeks after it launched its $50 unlimited plan. In comparison, the service lost 314,000 pre-paid customers in the fourth quarter of 2008. As Dow Jones (via WSJ) points out, if Boost can keep up this momentum, it could conceivably add more than 1 million customers in the second quarter, rivaling the growth of the country’s two dominant carriers, Verizon Wireless (NYSE: VZ) and AT&T (NYSE: T), which added 1.3 million and 1.2 million subscribers in Q1, respectively.

Of course, pre-paid customers aren’t as valuable as those on contracts, and it remains to be seen what impact the $50 plans have on Sprint’s profit margins, but prepay is one of the industry’s remaining bastions of growth, or as Boost head Matt Carter told Reuters, the service has not yet signed up “all the low hanging fruit.”

To keep the customers coming, Boost plans to open up 50 new stores by the end of 2009, which would take its total to 53 outlets. Carter told Reuters, “What’s important to us is street visibility. We think it’s going to help us quite a lot.” One thing Carter is worried about, however, is the possibility of a price war in which $50 unlimited plans, now being offered by its pre-paid rivals MetroPCs and Leap, might sink even lower. Carter said that while Boost would not cut prices further, who knew what “irrational competitors” might be prepared to do.

• Interview: Sprint CEO Hesse: ‘We Still Have A Lot Of Areas For Improvement’
• Earnings: Sprint Beats Expectations As Low-Cost Boost Service Lures Customers