Here is an interesting article. It doesn't mention when she converted but it does mention that she abandaoned the Muslim faith at an early age and why:

http://www.cbsnews.com/stories/2005/03/11/60minutes/main679609.shtml

This one has more interviews and I believe they suggest she is an apostate and not necessarily an atheist (although I read an interview where she stated that she did not believe in God or Angels):

http://www.americanthinker.com/articles.php?article_id=4529


>>Hi Peter,
>>
>>>>The most famous muslim politician in Holland is Hirsi Ali. She has been critisizing many muslim for beeing inhumane against woman. She has been threatened many times and she now is protected since the
>>
>>>Sorry Walter, but Ayaan Hirsi Ali is an atheist since three years. :)
>>
>>Ouch, I though she became atheist for a while and returned to the islam again soon after. Well, at least she is a former muslim who entered politics before she became atheist.
>
>Where did you read this, i.o.w. where can I read this?