I wrote a beginners article on how to create your own extension methods in .NET 3.5 and then how to go about testing them using new built-in features in VS2008. Please go read the article at the Code Project : http://www.codeproject.com/useritems/ExtensionUnitTests.asp.
If you are interested in extension methods (like I am obviously) and plan on writing your own, I think the article is a good starting point. I give suggestions on when to use them and when I think it is not appropriate. I also talk about some unexpected behaviors that occur when using extension methods on variables that are null. It then walks you through a step by step guide on how to write unit tests for the extension methods using the new unit testing features of VS2008 pro.
Please check it out and comment on how I could improve it and please remember to rate the article.