permissions on tables

Forgive me if this is not the correct list for this type of question.

I thought I understood PostgreSQL's privileges well enough, but I'm
running into problems, so I must misunderstand something.

I have a website that I'm adding functionality to, and therefore need to
expand the database. The database already contains around 30 populated
tables with 1 group role (group_reader) and 1 user role (user_reader).
To all existing tables, I'd assigned PUBLIC and group_reader SELECT
privilege.

Everything is working fine.

Now, I created one more group role (called group_writer) and another
user role (user_writer) and make sure that user_writer is a member of
group_writer.

I then explicitly grant group_writer SELECT privilege on all tables. (I
know this is technically not necessary as PUBLIC has already been
assigned SELECT privilege.)

I created (tbl_batch) and deliberately decided to not grant PUBLIC
access to this table. Instead, I granted group_writer SELECT, INSERT,
UPDATE, and DELETE privileges to this table. Looking at the ACL list
for this table confirms this.

When I attempt to access this table as user_writer, I'm denied access.
I'm access this through Tomcat and verifying the connected user as
user_writer.

I shouldn't have to grant the PUBLIC group full access to this table as
well, should I?

From what I understand of the manual, a user's privileges are the SUM of
the privileges of all groups of which that user is a member. Therefore,
user_writer's privileges should be {SELECT, INSERT, UPDATE, DELETE} from
group_writer plus {} from PUBLIC, which should yield {SELECT, INSERT,
UPDATE, DELETE}.

BTW, I'm running 8.2.6 on WinXP x64 SP2.

Thanks,

Gord