http://www.wired.com/reviews/product/kindle2
Here we go! That was easier than I'd expected. It actually says 80% after 500 charges. Based on once per week charging, that would indeed work out to almost ten years. Of course, this _IS_ relayed from Amazon marketing folks.
Here's the key quote:
And it flaunts a more powerful built-in battery: Amazon claims that the Kindle lasts four to five days with the wireless on (I got 4.5 days in my first test) and up to two weeks with it off. After a week of limited wireless, my meter is around 50 percent. Amazon also says that after 500 charges, it will hold 80 percent of its original juice. That means that most users won't have to replace the battery (a $60 procedure) for about a decade or so.