Reading about how improvements in computers (twice as efficient every 18 months) have made batteries last longer & longer. That just gets cooler.
Right now, with wireless off, my Kindle 3 works for at least a month between charges. So in about 6 years if the pace continues, the Kindle might last a YEAR on one charge. And at that point, they can put a little solar cell like the little solar calculators have...it might never need to be plugged in at all.
Similar for the iPad or iPhone of the future, Apple could hide solar cells around the screen, or even just in the Apple shape on the back & you'd rarely if ever have to plug it in. (Though an iPhone in a pocket gets less light, so might need to leave it on a desk :)