Testability is important to today's developers, and without a mocking framework available for Silverlight developers today, it is possible, albeit with a little extra work, to create testable components.

Maurice de Beijer has blogged about his solution for creating a testable Flickr web service API. He's created TestableWebClient and TestableOpenReadCompletedEventArgs types to assist with this, and kept the abstraction simple and to-the-point.

I agree with Maurice's comments: it isn't perfect, it isn't awful; it isn't pretty; but hey, it does get the job done, and he's found a good solution for this today.

Maurice also previously posted his own introduction guide to Silverlight unit testing.

Just wanted to pass along, definitely check this out!