The SoapScope weblog explains how to use their product to see a soap envelope from .NET that's passed over an https service.
I used another method, without using a tracing product, and you can copy the concept from here:
Basically, you implement a SoapExtension that processes the soap message and you print the output during the AfterSerialize Stage. Fairly painful since you have to add an attribute to every method in the Web reference proxy classes.
However, I do get to see the SOAP output from my service.
Unfortunately, I also get an exception:
An unhandled exception of type 'System.Xml.XmlException' occurred in system.xml.dll