Handmark couldn't simply port its iPhone application to the Kindle platform anyway, because of the significant hardware differences. "The Kindle is quite a bit more limited from a user-interface perspective," Lackey says. "Control of the Kindle is very much page-based," with the user usually pushing buttons to move from one page to the next. "It doesn't use the scrolling metaphor like you're used to on the iPhone and other smart phones," he says. The Kindle also lacks a touch screen.

Amazon's Kindle faces competition from other hardware makers offering portable devices with a variety of low-power color screens. These devices could serve as e-readers--at least in part. But Apple's existing relationship with software developers who write games and other small applications for the iPhone likely makes it the biggest contender for the e-reader throne. E-Ink, the display technology used in the Kindle e-reader, is already viewed as limited, given that it is slow to refresh, doesn't offer color, and can't support video.

"If Apple unveils a multipurpose media tablet with support for digital books and periodicals, content from a deep and wide range of publishers, and a color display, it is likely to be Kindle's biggest, albeit indirect, competitor," says Kevorkian.

But James Belcher, an analyst with ABI Research in Portland, OR, who covers e-readers, says he doubts that an Apple e-reader would "crush" the Kindle. "The market is still young," he says. "Apps won't decide who wins the e-reader game."

Belcher adds that the Kindle's biggest strength is the number of quality books that are available for it. If Apple gets into the e-reader space, its biggest challenge may be negotiating for those same books, he says. "Publishers will be leery of a potential hegemony." They have made it clear, he says, that "they are not interested in having their digital business dominated by Apple a la iTunes."