Josh has a good post on the question of certitude in Catholicism. He notes how one so often hears Catholic apologists asking, "How can you know that what you believe is true without an infallible teaching authority?" and then proceeds to outline some of his problems with certitude in the Catholic Church. Pointing to what he views as contradictions between how one doctrine was explained back in the day and how it's explained today, he posits that a Catholic can never really know the truth of a doctrine, because the explanation he hears today might change tomorrow.
I've offered a few thoughts in the comments box... I think it's an important post that Catholics need to address, or rather, they need to address Josh's argument.