Eli Lopian, the Israeli guy who is now running TypeMock - one of the most powerful Mocking frameworks out there for.NET developers, opened a blog. I personally know the guy and he is a brilliant coder and architect - so it's worth listening to what he has to say.

For example. in this post he lists resources for and against using TypeMock  because of the design issues you can run into - you don't have to design for testability if you use it - you just intercept all calls to objects(even static ones) when you test. it's really quite scary and too easy - does not make you think at all.

I'm from the "Design for testability" side. Eli is not - and he always has interesting arguments. He even wrote a rather controversial article on codeproject talking about this issue. Read it to get both sides.