I'm writing a Python library to access Ubuntu One's REST API. (Yes, I know one already exists; this is a scratch-my-itch-and-learn-while-doing-it project.)
The library will be a relatively thin wrapper around the REST calls. I would like to be able to unit-test my library, without hitting U1 at all. What's the best practise standard for making this possible? 
At the moment each REST call is an explicit http request. I can't see how to mock that out, but if I create a (mockable) UbuntuOneRESTAPI class hiding those http calls I suspect it will end up including most of the functionality of the wrapper library, which sort of defeats the purpose.
Your cutting point is the HTTP requests.
Write a mock library which intercepts the sending of the HTTP requests. Instead of sending them, convert them into a String and analyze them to test sending code.
For receiving code, mock the response handler. Save a good response from the REST server in a String and create the HTTP response object from it to test your receiver.
Write a few test cases which create these requests against the real thing so you can quickly verify that the requests/responses are good.
How to test python library wrapping an external REST service (without hitting the service)