Amazon are making changes to how their API is used for affiliates:
Effective immediately, we are renaming the Amazon Associates Web Service as the “Product Advertising API.” This new name more accurately reflects the purpose of the API, which is to enable developers to advertise products offered on the Amazon sites and thereby receive advertising fees from us. In addition to the new name, signatures will be necessary to authenticate each call to the Product Advertising API. This requirement will be phased in between May 11 and August 15, 2009, and by August 15, 2009, all calls to the Product Advertising API must be authenticated or they will not be processed