After you posted this I got curious too but I couldn't find a change log. I emailed Amazon support and asked what changed and where I could find a release log and here's what they said:
Unfortunately we do not have a change
log detailing the changes made between
versions of the Amazon Appstore app.
I will forward your feedback on to the
Appstore team who will take them into
consideration. Customer feedback like
yours is what allows us to continue to
improve and provide a quality product
If you have any other feedback please
let us know via email or at
Thanks for contacting Amazon.
This is actually the most direct answer I've gotten. I've emailed them a couple other questions and got boilerplate responses that looked like they were for different questions.