And here's the direct download link: http://www.amazon.com/gp/kindle/mac/download/ref=amb_link_15...
(warning it'll start downloading a 9MB DMG file as soon as you open the page!)
addendum: looks like the app is partially derived from the iPhone source (it uses the iPhone's standard URL scheme for launching the app... try kindle://home. I'm still trying to figure out how to launch a particular book.
Also, the Mac version uses QT extensively, and appears to feature some Growl integration.