Due to a combination of technical, legal, and permission issues outside our control, we're going to retire the send-to-Kindle feature on 3/31. You'll be able to use the service through the end of the month. After that point, it will no longer work. We apologize for the inconvenience this may cause you.
Update: Books already sent to Kindle will not be affected by this change. You will be able to continue to enjoy them on your Kindle after 3/31.