Update 2: Finally there is a proper Kindle Calendar app that works like a proper calendar should without having to resort to workarounds like annotations. You can download it here!
Update: While we’re still waiting for fully functional Kindle Calendar application you can download this Kindle Calendar Book which comes close enough in terms of functionality.
Even before KDK was announced people wanted to use Kindle to organize their lives better. So “Kindle Calendars” were some popular eBooks that sold on Amazon Kindle Marketplace. With Kindle lacking any interactive features at that time these were merely eBooks with a page for every day of the week. You could then type notes for every day and these notes would be backed up to Amazon’s online servers.
While this provided security in case your calendar was lost, it would not show you your upcoming appointments in any easy way and it wouldn’t synchronize with any calendar services like Microsoft Exchange based corporate or personal mail servers, MobileMe, Google Calendars etc.
With Kindle SDK out all these features would be easy to implement. Also this app can very well be free since calendar isn’t likely to cause a lot of internet traffic unless your schedule is extremely busy.
I would bet that Calendar App would be among the first ones to appear.