San Francisco - An analyst has estimated that more than a quarter of U.S. consumers who purchased Apple's (NASD: AAPL) iPhone last year "unlocked" the device for use on networks other than AT&T's (NYSE: T), Reuters reports.

An analysis of Apple and AT&T sales figures by Bernstein Research analyst Toni Saccohaghi found about 1.45 million iPhones that had been sold in 2007, but not activated on AT&T's network.

Some 480,000 of these are thought to be in AT&T's store inventory, while the other one million are believed to have been unlocked for use on other networks -- representing 27% of all U.S. iPhone sales.

"Besides the financial implications, we believe the prevalence of unlocked iPhones presents a significant strategic dilemma to Apple," Sacconaghi posited.

The percentage of unlocked handsets could prove a burden to Apple in negotiations with carriers to be its exclusive iPhone partner; currently, the deal requires that those who purchase an iPhone in the U.S. sign up for a two-year contract with AT&T.

Apple last week described the number of unlocked iPhones as "significant," Reuters reports.

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