I've met a lot of free software developers. I have never met one whose reason for doing what he (alas, rarely she) did was 'I want to get back at Microsoft'.
No, it was always for the fun of it, to make better software, because sharing code is easier than locking it away, because you didn't lose everything when you changed jobs: hell, even for the paycheque alone (though that is thankfully rare). But essentially nobody I have ever met engages in a creative act like software development to oppose Microsoft. Lots of Slashdotters think that's why they do it, but as far as I can tell this is a symptom of distance from the code and testosterone poisoning, nothing more.