> The timezone on our boxes is set as Etc/UCT.
>
> => select to_char(current_timestamp, 'HH12:MI:SS TZ');
>    to_char
> --------------
>  08:46:20 UCT
>
> However "UCT" is not a valid input format:
>
> => select '08:46:20 UCT'::time;
> ERROR:  invalid input syntax for type time: "08:46:20 UCT"
>
> Is there perhaps an assumption somewhere that what comes out is valid to
> go
> back in?

Right.  You need to use a timezone that PostgreSQL recognizes.

This is by no means a new issue; people have noticed this requirement
since version 1.0.

http://linuxfinances.info/info/requirements.html

 Some users have reported problems that have been traced to their locales
indicating the use of some time zone that PostgreSQL did not recognize.

    * On AIX, TZ=CUT0 was unrecognized, leading to timestamps pulled from
system calls causing it to break.

      CUT0 is a variant way of describing UTC

    * Some countries' timezones are not yet included in PostgreSQL.

In any case, what commonly seems to be the "best practice" with Slony-I
(and, for that matter, PostgreSQL) is for the postmaster user and/or the
user under which slon runs to use TZ=UTC or TZ=GMT. Those timezones are
sure to be supported on any platform, and have the merit over "local"
timezones that times never wind up leaping around due to Daylight Savings
Time.