I agree that AWS is not a usual SOAP implementation, but I thought it worth noting that I used PHP5's native SoapClient object to access AWS and I had zero problems consuming the service. The only thing I would consider awkward about it is their documentation. They don't make it very clear very early what types of parameters to pass to the functions, and if they're arrays, which elements.
Yes, I realize there are a bajillion PHP 4+ sites out there that can't use that native function, but really now...you've had over 2 years to convert your site to PHP5. And you know you should. I sympathize with PHP4 guys, but only so much. We're almost at PHP6 now, for cryin' out loud. </rant>