function with security definer

Hi
I have a lot of users for my database, but I don't want to create each
of them as postgres user, so I tried to do this:
1. Create only postgres 3 users: admin,user,nobody
2. Login into database _only_ as nobody and execute only 1 function -
make_login.
3. This function checks user login and password
4. If everything is ok, function switches from nobody to either user or
admin.

Here is a body of function:

CREATE OR REPLACE FUNCTION make_login(varchar,varchar) returns integer AS'
DECLARE
login_ ALIAS for $1;
passwd_ ALIAS for $2;
access_level integer;
BEGIN
select into access_level access_level from users
where login=login_ and passwd=passwd_;
if not found then
return -1;
end if;
if access_level=1 then
set session authorization ''user'';
end if;
if access_level=2 then
set session authorization ''admin'';
end if;
return 1;
END;
' LANGUAGE 'plpgsql' SECURITY DEFINER;

Well, it doesn't work :-(
ERROR: permission denied

Why? Everything looks ok. User nobody has acl to execute this function.
Function is created by database owner. What else do I need?

Regards,
Tomasz Myrta