alter user does not changes password

Hi all:
I know it's a silly question, but I've googling around and
searching mailist archive with no answer to my question:
I'm using self compiled PostgreSQL 7.2.1 in a RH 7.1 box.
As described in http://www.postgresql.org/idocs/index.php?sql-alteruser.html
I'm trying to change a user's password as:
 alter user camara with password 'canabis!';

but when I try to connect:

$ psql -U camara dbcamara;

User "camara" logs with no password!!!! If I use -W option of psql
It prompts for password and I can enter whatever I want... it accepts!
It must be a feature.. not a bug... I know I'm making a mistake.. but.. 
where??

bests regards

-- 

[]'s
Lucas Brasilino
brasilino@recife.pe.gov.br
http://www.recife.pe.gov.br
Emprel -    Empresa Municipal de Informatica (pt_BR)    Municipal Computing Enterprise (en_US)
Recife - Pernambuco - Brasil
Fone: +55-81-34167078