According to the recent changes in Amazon all requests to the Product Advertising API will need to be signed: signatures will be necessary to authenticate each call to the Product Advertising API or they will not be processed.
Read more about latest Amazon update.
This change will affect all previously generated Amazon feeds. You should re-generate all your feeds using our updated Amazon RSS Feeds Generator by August 15, 2009.
Starting from August 15, 2009 you will have to implement signature in your Amazon RSS feeds. To generate and embed your signature to the feed you will have to use your "Amazon AWS Access Key ID" and "Amazon AWS Secret Access Key". If you haven’t obtained your keys yet, please sign up at Amazon.com.
RSS Ground team