I've got the latest firmware which allows you to tweet a review once you've finished reading a book.
The only problem I have, is that this doesn't work with non-Kindle books. I've got some .mobi books (which I've converted from .epub)
The sharing function appears, and I get the "message shared" popup - but nothing appears on Twitter.
I know that I'm OAuth'd correctly because other - Kindle bought books - allow me to share.
Is this just a limitation of the Kindle? Is there any way around it?