Chris Walters has an article about this on his Booksprung site today:
Today Amazon rolled out a new online storage service called Cloud Drive. I immediately wanted to see whether it would be useful to Kindle owners, so I tried uploading and downloading some mobi-formatted ebooks to it. The short answer is that it works, although it’s not the most user-friendly interface on the Kindle.
Take a look if you want to see the details of how it works.
