Permissions with multiple groups...

I'm receiving permissions denied message when I think I shouldn't.

I have three users cib_unauth, cib_auth, and cib_admin.
I have two groups cib_all and cib_trusted. The first group contains
all the users, the second group contains the latter two.

I'll grant permissions like
REVOKE ALL ON sometable FROM PUBLIC;
GRANT SELECT ON sometable TO GROUP cib_all;
GRANT INSERT ON sometable TO GROUP cib_trusted;
GRANT UPDATE,DELETE on sometable TO cib_admin;

However I'm getting permission denied messages on sometable when I'm issueing
selects as cib_admin.

\dp sometable
sometable | {"=","nielsene=arwR","cib_admin=w","group cib_all="r","group cib_trusted=a"}

Connecting as cib_auth has the same problem, but connecting as cib_unauth
works.

I haven't found anything in the documentation about problems with a user being
a member of two groups. However, I suspect I might be slightly abusing
user groups?

Eric