Currently Being ModeratedJul 18, 2012 4:16 PM (in response to gadgetmax)
No such notification mechanism exists. If customers re-download the book, they will get the new version, but there is nothing to tell them that a new version is available.
The next best thing you can do is to publicize new versions elsewhere, such as on your website. For my book, I invited readers to contact me via email so I can add them to notification list for updates.