MSDN just published my article on API testing:
"If you ask several testers how to write good API tests you will get several different answers. Not only are peoples' styles and approaches different, but often API sets will lend themselves to various methodologies. That being said, my goal here is to describe some of the practices that have worked well for me while testing the Xbox Operating System. Hopefully the information detailed here will be beneficial and useful in your own tests."
If you haven't swung by recently, you should check out the Tester Center which hosts a whole bunch of testing related content.