One thing worth mentioning about the audio capability of the Kindle Keyboard:
One of the perks of the new firmware is that it now syncs audio and text
One of the things I hate about Audiobooks is how easy it is to lose your place -- no one outside the recording industry seems to have perfected bookmarking audio tracks because it's never been important for music (unless you're listening to Morton Feldman's six-hour string quartet
With the Kindle Keyboard, you can now bookmark the page where your listening/reading ends and pick up your place in the recording with it. You can also use text-to-speech and the page actually turns as the spoken text progresses.
If I had a child or knew someone who was just learning to read, I'd be in love with this feature. Perhaps not important to you, but it does take the KK's seemingly irrelevant audio capability and turn it into a benefit which no other e-ink reader currently offers.