According to a market study by research firm Canalys, more iPhones were sold in the fourth quarter in the United States than Windows-based mobile phones. The Canalys study estimated the iPhone to have 28% of the converged device market in the United States for the fourth quarter, while Windows Mobile phones had 21%. Research In Motion (RIM) had the largest share of the U.S. market with 41%.

Outside the States, however, Apple squeezed into third place for the smartphone market. Nokia, the world’s top mobile phone maker, was tops for the converged devices category as well with 53% of the market share. RIM’s share grew considerably, but is still way behind Nokia with 11% worldwide and Apple came in third, just ahead of Motorola with 7%.

Still, for debuting a brand-new product, 7% of the worldwide market is not a bad show.

Analysts with Canalys say Apple’s success is striking, but the challenge will be to keep up the momentum. With only one device, Apple may struggle to maintain its current market share despite the iPhone’s popularity.