Behavior of "at time zone"

Hi there,
         I'm looking to use the "at time zone" language feature, however the results below don't really agree with my
expectations. Is this a bug in 8.4 or am I misinterpreting the results?  If it is a bug, has it been fixed in the
developmentreleases? 


psql (8.4.0)
Type "help" for help.

ispdb_br=> set timezone to '-2';
SET
ispdb_br=> select now();
              now
-------------------------------
 2010-01-29 11:26:22.833697-02
(1 row)

ispdb_br=> select now() at time zone 'BRST';
          timezone
----------------------------
 2010-01-29 11:26:30.521964
(1 row)

ispdb_br=> select now() at time zone '-2';
          timezone
----------------------------
 2010-01-29 15:26:47.010222
(1 row)

ispdb_br=> select now() at time zone '+2';
         timezone
---------------------------
 2010-01-29 11:26:49.55412
(1 row)


Regards,
Andrew