Amazon may have sold 500,000 Kindle devices before selling out during the holidays, blowing past an earlier estimate of 378,000 units, according to Citi analyst Mark Mahaney. Although the company has not yet disclosed the exact number of units shipped, the integrated EVDO antenna in each Kindle has provided an alternative gauge. Sprint's financial filings indicate third-quarter distribution of 210,000 "certain wholesale devices" that were activated by wholesale partners prior to being sold to end consumers.

The device was nicknamed the "iPod of reading" when Amazon first released it onto the market. If the sales estimates are accurate, the devices could be showing early strength that rivals the original iPods. In comparison, Apple sold less than 400,000 media players through the first full year of availability.

The company currently offers roughly 230,000 titles for the Kindle, which sold out shortly after an endorsement by Oprah Winfrey. Rumors suggest that an updated version is ready to be released as early as next week.