There are several “solutions” that provide stubs that can be used in unit testing Google AppEngine applications, patient including something called a “testbed” which is part of the API itself. But the problem with these is that they provide functional bits of API implemented on your local environment just like it would work on a deployed AppEngine application.
It sounds quite good to have a local personal instance of something similar to the datastore you get in deployed applications, but unfortunately for the urlfetch service it is not exactly what I was looking for in tests.
The thing I need is an object that will not urlfetch anything, will not access the network at all. The requirement in this case is an object that I can tinker with its state before and after my own methods have used the urlfetch facility. After a lot of digging in the current implementation of the stub, I ended up writing a very simple mock for this myself. It is far from perfect, but its a start.