I came across an article in ComputerWorld earlier today, and I just got confirmation of the story from a Kindle PR rep.
Amazon will be showing off the full blown K4Web tomorrow. It's only a demo, though. You can read the ComputerWorld article if you want, but they don't have any more information than I do.
It's a demo. Whatever. Wake me when it's launched.
Update: Just in case you're interested, here's the email I received:
Kindle owners and app users can already choose from millions of books and sync their books, bookmarks, notes, and highlights across Kindle, iPad, iPhone, Blackberry, PC, Mac, and Android devices. Tomorrow we'll be demoing a new version of Kindle for the Web that enables users to read full books in the browser and any website to become a bookstore offering Kindle books-stay tuned.