Today, I noticed that Amazon added a new feature to their book product subscription web page that allows one to "Ping" the publisher for a Kindle version. Thank you Amazon.
For example, I would like a Kindle version of John Grisham's The Brethern. So, just click on the link and if you look to the left side, below the book cover, you should see a new box that reads as follows.
Please tell the publisher: I'd like to read this book on Kindle
Let the pinging begin.