The strange thing is, the key I'm using now has been my key for months, and the Amazon plug-in had been working fine... it only suddenly stopped displaying properly sometime in July. So I assume if there was a problem with a special character, it would have been causing me an issue all along? The only special characters I see in mine are / and +.
I haven't been using the sidebar but rather the tags within a post (of the "[amtap amazon:asin=B003EELV1Q]" variety). Again, they were showing up all along, then stopped. I think this was even before I upgraded my WordPress version, so I don't have the impression that the WP upgrade itself broke anything. Even with the debug option checked, my source for the posting shows no reference (commented or otherwise) to the Amazon tag. It doesn't render the "[amtap amazon:asin=B003EELV1Q]" in plain text, so it clearly recognizes it as something special, but there is nothing there at all where it should be appearing in the source. Strange!
I'm looking to see if there's anything weird on Amazon's site re: my account, but no luck so far.