postgres=# set timezone='+03';
SET
postgres=# select now();
now
-------------------------------
2011-03-18 11:20:29.696671+03
(1 row)
postgres=# set timezone='+03:00';
SET
postgres=# select now();
now
------------------------------
2011-03-18 05:20:35.46008-03
(1 row)
(checked with 8.4 and 9.0)