Timestamp/Timezone - does this make sense?

mvh=> set time zone 'UTC';
SET
mvh=> select now();
              now
-------------------------------
 2007-02-13 03:37:35.660652+00
(1 row)

mvh=> select timestamp with time zone '2007-01-01' at time zone
'America/Los_Angeles';
      timezone
---------------------
 2006-12-31 16:00:00
(1 row)

mvh=> select timestamp '2007-01-01' at time zone 'America/Los_Angeles';
timezone
------------------------
 2007-01-01 08:00:00+00
(1 row)

Where does that extra 8 hours come from?