No wonder Amazon.com (Nasdaq: AMZN) and Barnes & Noble (NYSE: BKS) just announced price cuts for their respective e-book readers. Both companies seem to be running scared from Apple's (Nasdaq: AAPL) iPad, which just plowed through another sales milestone.

In only 80 days since its release, Apple's latest tantalizingly shiny gadget has sold 3 million units worldwide. Well, not quite worldwide -- the device still has yet to launch in a host of countries including Mexico, Ireland, and New Zealand, which should further help boost its sales figures.

Faced with Apple's e-book-reading, Web-surfing, movie-streaming, game-playing juggernaut, Amazon in its ads is reduced to saying the Kindle can be easily read in direct sunlight (unlike some tablets it could mention). Gosh, I don't know about you, but I'm dazzled.

In fairness, the Kindle's E-Ink screen reportedly does beat the pants off the iPad for readability in bright light. Furthermore, lots of happy customers love their Kindles. But the iPad's greater versatility, and the undeniable gloss of Apple's high-end cachet, seem to be powering it to far faster sales growth.

Amazon still hasn't released specific sales figures for the Kindle, but in January of this year, tech pundits estimated that the device had sold 3 million units. However, it took Amazon more than two years after the Kindle's launch in November 2007 to reach that mark, even though the Kindle offers free wireless access and has consistently cost less than even the least expensive iPad.

The iPad's swift success suggests that Steve Jobs and his team have another winning device on their hands. That not only makes life harder for Amazon and Barnes & Noble, but also increases the pressure on Google's (Nasdaq: GOOG) still-hypothetical Android-based tablet and Hewlett-Packard's (NYSE: HPQ) back-to-the-drawing-board Slate. To make a dent against the iPad -- and the accumulated might of Apple's entire mobile ecosystem -- these devices will have to be truly extraordinary. May the best tablet win.

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Fool online editor Nathan Alderman's dad really, really loves the Kindle. Nathan holds no financial position in any of the companies mentioned here. The Fool's disclosure policy is easily readable in anything short of total darkness.