However, the code is really just the GPL'd Linux libraries and drivers (Debian Linux is the common speculation), and it's been available since 2007. The e-book reader part is in no way part of the source package.
While no license file is attached, the folder the code unpacks into is called "gplrelease," which might be a massive hint. Not to mention all that's in there is what would be required for them to release legally for the GPL portions.
Jeff Bezos of Amazon had hinted recently at a conference that Kindle books and Kindle readers were, to him, two separate businesses. Releasing a Kindle iPhone app was certainly one hint that he was serious. Another was his statement that he intended to give the Kindle device team competition by making the Kindle books available on even more mobile and computing devices.
But entirely releasing the Kindle source doesn't appear to be the way they're going to do it just now. Who knows, maybe later if Amazon decides it would rather offer their content on more devices and get out of the hardware business.