The concept of mocking web services for testability took a little while to sink in for me. A big part of it was that my job doesn't see me consuming web services all that often, but I had an opportunity to give it a shot with SOAP. I found that I learned a lot more about testing in general having worked through this. I used SoapClient and wrapped it, so here's a little bit about some of things I learned. Hopefully you don't have to work with SOAP, but if you do you can test it pretty easily.
He walks through the mocking of the client itself and how he handled it's ability to translate function calls into SOAP method calls (using "__call") and how he mocked that. He also makes the suggestion that you actually wrap the SoapClient inside of another class rather than trying to mock the actual SoapClient. He also touches on the testing of exceptions that might be thrown by the service and how he tested those using his wrapper class.