Here is what Amazon says on the subject:
• Shop the Kindle Store for over 750,000* Kindle books, including New Releases and 107 of 111 New York Times Best Sellers. Over 550,000* of these books are $9.99 or less, including 73 New York Times Best Sellers.
• Choose from thousands of free classics, such as Pride and Prejudice or The Adventures of Sherlock Holmes.
• Get free book samples – read the first chapter free before you decide to buy.
• Customize your reading experience by choosing the background color, font size, and either portrait or landscape format.
• Personalized book recommendations on your Kindle app home screen and the ability to send a book suggestion to a friend from any book in your library without leaving the app
• Simply tap on either side of the screen or flick to turn pages.
• You can also read your Kindle books on your Kindle, Kindle DX, PC, Mac, Android phone, iPhone, iPad and BlackBerry. Our Whispersync technology syncs your last page read, notes, bookmarks, and highlights across devices, so you can pick up where you left off.
• Selection and price of books in the Kindle Store may vary by country.
This news bodes well for WP7's future, since it's vital for every mobile platform to have some of the so-called "killer applications" - and nowadays Kindle is a must have app for every mobile OS.
source: Amazon and Marketplace Browser via Engadget