Amazon in a press release:
Since April 1, for every 100 print books Amazon.com has sold, it has sold 105 Kindle books. This includes sales of hardcover and paperback books by Amazon where there is no Kindle edition. Free Kindle books are excluded and if included would make the number even higher.
This is all about the Kindle. There are quite a few people in my life that surprised me by owning a Kindle this year — I doubt Kindle ownership will be uncommon by this time next year. The device is too cheap and too good for people to not buy it at this point.