A friend turned me on to this post by James Whittaker. I didn't know he had a blog so now I'm excited to read it. He has a lot of really interesting things to say on testing so I encourage you to read his blog (now linked on the left) if you are intrigued by testing.
Microsoft prides itself on the advanced state of its testing operations. This leads to the inevitable question, "If Microsoft is so good at testing, why does your software suck?" James Whittaker was once a person who asked this question and now that he's at Microsoft he is in a good position to try to answer it.
James gives basically three reasons: