True if you need to carry a PC everywhere. But what if you don't? And besides, the advantage to the Kindle and the Sony is that they are built as reading devices, with e-ink screens to maximize eye comfort. I don't see them wiping out the e-reader market, especially not if the Kindle keeps free web access. If that were true, I think laptops would have become e-readers by now.
But as technology advances, we could definitely see the all inclusive device in our future—phone, GPS, web browser, e-reader, all in one. Star Trek tricorder here we come!