Comparing printers and eink is complicated by the fact that printers can only display pure black and white, while the eink can anti-alias with 16 shades of gray.
At the normal distance range that you hold a Kindle while reading, the human eye can perceive the anti-aliasing as increased resolution, allowing much LESS than 300 DPI to look great, while on a pure black and white printer, 300 DPI looks crude in comparison and you really need 600 DPI or better.
So anti-aliased eink displays have a higher "effective" resolution than their technical DPI ratings would otherwise indicate.
References:
http://www.ncbi.nlm.nih.gov/pubmed/9304755
Quote:
Anti-aliasing is a technique for improving spatial resolution at the cost of luminance resolution. Dithering is a technique for improving luminance resolution at the cost of spatial resolution. These techniques are applied to the testing of visual function in the 'Freiburg Visual Acuity Test': by employing anti-aliasing, spatial resolution was improved.
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Interesting anti-aliasing tutorial:
http://www.beatstamm.com/typography/RTRCh2.htm