This guy had my same problem. I'm recording this here for posterity's sake.
Eureka! SoapExtension problems solved! - Jan Tielens' Bloggings: "Maybe you've read my post about the problems I had while using a custom SoapExtension class. Thanks to an anonymous reaction, I found the sollution!! I was testing the webservice by using the default test page from Internet Explorer, but the testpage in IE uses the GET protocol, not the SOAP protocol. So there was nothing wrong with my code (I started believing I really sucked ;-), I only had to make a very simple test application. Thanks again to the person who submitted this tip!"