Rav Aharon Lichtenstein considers the general question of whether
or not one can lose his status as a Jew in "Brother Daniel and the
Jewish Fraternity" (Judaism, v. 12 (1963) pp. 260-280) His conclusion
is that in a very real sense, one can lose one's status as a Jew by
converting to another religion. Unfortunately, he does not spell
out the Halakhic consequences.
There is also an interesting teshuvah (legal opinion) by Rabbi David
Tzvi Hoffman (Melamed l'Hoyil) from turn of the century Germany. He
is confronted with the case of a Jew who registered with the German
government at "confessionless", i.e. he denied Judaism without conveting
to a diferent religion. The conclusion of this teshuvah is that the
Jew in question must go to the Mikveh in order to regain his status in
the Jewish community. I don't have the exact reference handy, and the
teshuvah is fairly difficult Hebrew. However, I can get the exact
reference or a photocopy for anybody who is interested.
Hope this helps ...
Adam Szpiro (firstname.lastname@example.org)