I've just started using the SOAP Client module in Drupal to handle SOAP requests. It's a great little tool. If you set it to use the nuSOAP PHP libraries (instead of core) it even handles Microsoft datasets, which some Microsoft-based web services will return.

There is one small problem. Documentation is approximately zero! The only assistance offered is "look at the code of the admin test form", which is great for simple name/value pairs of data, but doesn't tackle complex types.