There's a new update in iTunes today for the the Kindle app for iDevices. According to the changelog, it fixes a problem that has yet to officially occur:
This is a fix for a Kindle issue with the upcoming iOS7 OS upgrade that may cause customers to have to re-register and re-download books from Amazon. Please download and install - this is a required update before upgrading to iOS7.
Amazon has clearly been testing the Kindle app on the beta releases for iOS7, and that they found a problem. Today's update fixes the problem before it breaks everything.