The Kindle has a built in cell receiver so that you can wirelessly browse, preview and purchase books using one-click.
Typically ebook readers are larger than PMP's with larger screens. If you're going to read for a few hours, then the screen of a typical PMP becomes pretty eye-straining (due to size, contrast, and brightness).
If you read some of the Kindle discussion groups, you'll see people complaining about "no backlighting", and then a few days/weeks later they will be lauding the fact that it has no backlighting.
A cheap casioppeia allows me to read ebooks around 2001.
A revolutionary portable reader that wirelessly downloads books, newspapers, magazines and blogs to a crisp, high-resolution electronic paper display that looks and reads like real paper, even in bright sunlight.