When Jason asked "Why are Safari and Sherlock two different applications?" it made me realize that what I want is a desktop application, similar to Andre’s Konstructor, that understands SOAP as well as XML-RPC and can query a WSDL file to find out what kinds of input the web service requires.

Then, based on the type of input requested, it would display the appropriate widgets (checkboxes for true/false, drop-downs for selection lists) and arrange them on a form according to an editable set of rules. The end results wouldn’t be pretty, but you might end up with a bunch of applications that assemble themselves around web services.

If you could write the whole thing in XUL/javascript, it could run within any Mozilla-based browser.