As inspired by this thread: http://markmail.org/thread/v3qptyjsrvsxi2j2
Usually in an integration test we want the "real" implementations of our domain services. That's certainly the case for repositories etc that connect to "our" database.
But for domain services that connect to the outside world (eg hit an external web service), it'd be useful to be able to replace the real implementation with a mock, and have this mock be used in the running system instead.
That mock would probably need to have a fairy liberal set of expectations, eg (in JMock) use allowing() or ignoring(); so I can see a bit of experimentation might be needed to make this work smoothly. But it undoubtedly would be valuable.