def some_method_that_uses_a_soap_web_service
storage = SOAP::WSDLDriverFactory.new(STORAGE_WSDL).create_rpc_driver
...
storage.wiredump_dev = STDERR
response = storage.DoSomething(...)
end

When the DoSomething web service method is called, the Ruby SOAP library will print out the request and response to STDERR.

It is invaluable. Trust me!

Follow-up (8/13/2009): A friend pointed me at Handsoap, a library for creating SOAP clients in Ruby, which is an alternative to SOAP4R. It apparently does not suck. YMMV.