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
and experience.
If you have any other feedback please
let us know via email or at
1-866-749-7771
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.