pgsql-bugs@postgresql.org writes:
> it's not null, so shouldn't it be retrieving as '00:00:00:00:00:00'?
Yeah, due to some brain-fade on the part of the original implementor,
the code went out of its way to special-case 00:00:00:00:00:00 as
being represented by an empty string. This was agreed to be broken
and we've changed it for 7.2. If you like you can drop the 7.2
version of the file mac.c into 7.1; see
http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/mac.c
regards, tom lane