Should we throw error when converting a nonexistent/ambiguous timestamp?

It's DST transition season again, and that means that we're getting the
usual quota of questions from people who don't quite understand how
DST-related timestamp arithmetic works, and whose incorrect code seems
to work until exercised during a transition interval. We've got this
one from a guy who got bit by converting a nonexistent local time:
http://archives.postgresql.org/pgsql-general/2010-03/msg00590.php
and last week we had one from an Aussie who was getting bit by the
behavior for ambiguous local times at the other end of the cycle:
http://archives.postgresql.org/pgsql-general/2010-03/msg00459.php

I'm starting to think that maybe we should throw error in these cases
instead of silently doing something that's got a 50-50 chance of being
wrong. I'm not sure if the "assume standard time" rule is standardized,
but I think it might be better if we dropped it. Thoughts?

regards, tom lane