On Tuesday, Citi Investment Research analyst Mark Mahaney offered an educated guess - that Amazon sold 500,000 Kindles last year. That is higher than his previous expectation of 380,000.

Mahaney bases his latest estimate in part on a regulatory filing from Sprint Nextel Corp., which is a key player since the Kindle downloads books wirelessly over Sprints network. The filing notes that 210,000 of “certain wholesale devices” were activated on Sprints network during the July-September period by “wholesale partners prior to selling the device to the end customer.”

Mahaney said in an interview that this is the first time Sprint has disclosed such a figure, and he believes it refers directly to the number of Kindles that were purchased during that three-month period. Sprint spokesman James Fisher said the company had no comment.

Amazon has not released sales figures for the $359 Kindle - which debuted late in 2007 - though it said last fall that it had sold out of the devices after an endorsement by Oprah Winfrey. In the fourth quarter, the company made another 45,000 book titles available for the Kindle, bringing the total to 230,000.

The company is widely believed to be releasing a new version soon.

In his note to clients, Mahaney estimated that revenue from the Kindle could rise to $1.2 billion by 2010 - which he thinks would be 4 percent of Amazons revenue for that year.

Amazon spokesman Drew Herdener would not address the accuracy of Mahaneys estimate.

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