Amazon has announced “Kindle Matchbook,” coming in October. If you have bought a book new from Amazon, you can get the Kindle version at a greatly discounted price. They write:
For thousands of qualifying books, your past, present, and future print-edition purchases will soon allow you to buy the Kindle edition for $2.99, $1.99, $0.99, or free.
(Thanks to a Facebook friend for the tip.)