Apple's iPhone had a breakthrough year. The $99 2G iPhone and the $199 iPhone 3GS gave AT&T ( T) both a low road and a high road to blockbuster sales. The numbers. Apple sold 7.4 million iPhones in the most recent quarter and gained four percentage points of market share worldwide to climb to third place with 17% of the market. The iPhone is in its third year, its final year as an exclusive to AT&T. As international expansion continues, there have been questions about what service provider will land the iPhone next in the U.S. Until Monday, the presumed next destination was Verizon, but Weisel analyst Doug Reid predicts a detour to T-Mobile. The stock. Apple shares are up 133% this year as the Cupertino, Calif. gadget shop continues to make magic with consumers and investors. The big challenge: How Apple will keep that momentum going.