Amazon is selling more Kindle versions of books in the US than hardback and paperback books combined

"Amazon began selling hardcover and paperback books in July 1995. Twelve years later in November 2007, Amazon introduced the revolutionary Kindle and began selling Kindle books. By July 2010, Kindle book sales had surpassed hardcover book sales, and six months later, Kindle books overtook paperback books to become the most popular format on Amazon.com. Today, less than four years after introducing Kindle books, Amazon.com customers are now purchasing more Kindle books than all print books - hardcover and paperback - combined.
"Customers are now choosing Kindle books more often than print books. We had high hopes that this would happen eventually, but we never imagined it would happen this quickly - we've been selling print books for 15 years and Kindle books for less than four years," said Jeff Bezos, Founder and CEO, Amazon.com. "In addition, we're excited by the response to Kindle with Special Offers for only $114, which has quickly become the bestselling member of the Kindle family. We continue to receive positive comments from customers on the low $114 price and the money-saving special offers. We're grateful to our customers for continuing to make Kindle the bestselling e-reader in the world and the Kindle Store the most popular e-bookstore in the world."
Recent milestones for Kindle include:
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.
So far in 2011, the tremendous growth of Kindle book sales, combined with the continued growth in Amazon's print book sales, have resulted in the fastest year-over-year growth rate for Amazon's U.S. books business, in both units and dollars, in over 10 years. This includes books in all formats, print and digital. Free books are excluded in the calculation of growth rates.
In the five weeks since its introduction, Kindle with Special Offers for only $114 is already the bestselling member of the Kindle family in the U.S.
Amazon sold more than 3x as many Kindle books so far in 2011 as it did during the same period in 2010.
Less than one year after introducing the UK Kindle Store, Amazon.co.uk is now selling more Kindle books than hardcover books, even as hardcover sales continue to grow. Since April 1, Amazon.co.uk customers are purchasing Kindle books over hardcover books at a rate of more than 2 to 1."

Source:  Press release from Amazon, 19th May 2011