It’s broken. Don’t install it!
Edit: Apparently the auto-update isn’t downloading this version. it’s only the full manual install that is broken. So basically this means you shouldn’t go to Amazon and download it.
Update: There’s now a growing list of people over on MobileRead who are also having trouble with the latest update.
Second Update: And Andrys of KindleWorld pointed me at a number of complaints over on the official Kindle forums.