From their documentation
Amazon.com Web Services offers developers the choice of XML/HTTP or SOAP to access information in our catalog and databases. Which method should you use? Mostly, it depends on what you are the most comfortable with. There is significant industry debate over which Web Services method is the “best”; at Amazon.com, we decided to take a neutral approach and offer access via both methods.
Apparently they tested the same source code with both Sun’s JAX PAK and Apache Axis; and they have a Perl client using SOAP::Lite. As with Google, It looks like Amazon.com has done their homework, providing examples of the protocol as it goes across the wire, machine readable metadata in the form of WSDL, and testing with a number of cross platform implementations.