Thanks, jewbiquitous and TalSh.
Jewbiquitous, your mention of the Anabaptists was pretinent and instructive, hence i've given you one thumb up.
As for TalSh, i appreciate your willingness to answer my question, but i'm afraid your answer is merely an unsubstantiated personal opinion, that has taught me nothing new. Hence i've given you one thumb down.
I'm leaving this thread open for the time being, since i'd still like to learn more about the way Islam deals with this issue. I'd also like to know whether there is, or was, any "Anabaptist" Jewish sect, that denies the Jewishness of a person, unless he/she converted to Judaism consciously and willingly (as an adult).