When the Kindle debuted in 2007, it included the ability to download books, newspapers and magazines over Sprint's third-generation EV-DO (Evolution-Data Optimized) network. The cost of using the network was built into the price of the content, so consumers could take advantage of a 3G network without worrying about carrier choice, contracts or monthly data plans. Executives across the U.S. mobile industry pointed to the arrangement as a sign of a future in which carriers could add new revenue sources and device makers could find new routes into consumers' hands.

Sprint was a natural choice for Amazon, having led the MVNO (mobile virtual network operator) business for years by letting third parties such as Samsung and EarthLink's Helio and The Walt Disney Co. buy wholesale capacity on its network and offer their own mobile services. Many MVNOs eventually sank, with observers pointing to several possible causes, but Sprint had experience in opening up its network for new uses.

However, other carriers soon began exploring new ways to use their infrastructure. As Google weighed in on the U.S. Federal Communications Commission's 700MHz spectrum auction, demanding part of the frequencies be opened to any device, Verizon Wireless said it would open up part of its network. Both Verizon and AT&T, along with Sprint, lately have played up the importance of "machine to machine" services that go beyond users making voice calls or browsing the Web.

Earlier this month, looking to expand on the Kindle phenomenon in the U.S., Amazon announced it would sell the device internationally. Eyeing the bulk of mobile networks and potential carrier partners around the world, the company introduced a new version that uses HSPA (High-Speed Packet Access), a form of 3G based on GSM (Global System for Mobile Communications) technology. AT&T, the dominant user of that system in the U.S., provides the book downloads on the new device.

Sprint's 3G system, EV-DO (Evolution-Data Optimized), was left behind except on the Kindle DX, a larger and more expensive model that is still only sold in the U.S. (Owners of the original device still have access to Sprint's network, as before.)