You learn the darndest things on the Web

From the soc.culture.jewish FAQ file (available here).

  Subject: Question 6.17: Must Jews use wine?

  Answer:

  Actually, grape juice can be used, even if it's just because
  you like it more.

  By the way, Rav Moshe Feinstein ruled that Catholics are
  allowed to use grape juice for the eucharist. To be more
  precise, R' Dr Aaron Twersky is both a rabbi (from a long
  line of Chassidic Rebbeim, but I mean "rabbi" in the sense
  of having a synagogue, not the head of a community) and a
  psychologist [sic] who works in a substance abuse center. One of
  his patients was a preist [sic] who was being cured of alcoholism.
  This means he can't drink even a drop of wine again; a
  problem for a priest who must take communion. His self-
  control just isn't reliable. So what is he supposed to do
  during Mass? He voiced this concern to Rabbi Twersky, who
  asked why he didn't just use grape juice. The priest asked
  his cardinal, and eventually the problem made it all the way
  to Rome. Can grape juice be used for communion? Well, the
  Vatican heard that we consider the grape juice a kind of
  wine, but they wanted to know more. So the question went
  back to the priest and Rabbi Twersky. R' Twersky sent back a
  citation of a responsum of R' Moshe Feinstein, allowing the
  use of grape juice for the seder. The Vatican concluded that
  if grape juice is okay for the seder, then it was usable for
  the Last Supper, and therefore when Jesus said at that meal
  "this is my blood" he meant grape juice too.

Twersky is a psychiatrist, by the way, not a psychologist.