Strange Authentication Problem

Hi,

i have a strange authentication problem with postgresql v8.3 on mac os
x server (10.5).
my pg_hba.conf contains only the following 4 entries:

local all all md5
host all all 127.0.0.1/32 md5
host all all ::1/128 md5
host all all 0.0.0.0/0 md5

if i connect from local machine with 'psql -U testuser testdb' or
'psql -h localhost -U testuser testdb'
psql prompt me for a password and i get in - looks good so far. next
step was to try connect from
another machine on another network.
with 'psql -h <remote hosts ipaddress> -U testuser testdb' i get a
prompt for a password and if i give
the correct password i can login and do something more or less
useful ;-)

but now my problem. if i connect with the full qualified domain-name
to the remote machine, e.g.
'psql -h testdb.example.com -U testuser testdb' i get no password
prompt, instead psql logs me
in without the need of a password and i have full access to the db!

hm. it seems to be a problem with md5. if i change the last line in
pg_hba.conf to the following:
host all all 0.0.0.0/0 reject
and reload the servers configuration, then it will reject my
connection if i use ip-address or the
domain-name of the remote machine. if i change it back and reload the
configuration, then i
can login without password again when i use the domain-name of the
server and can login
only with password when using the ip-address.

if i try the -W switch of psql and connect with domain-name i get the
password-prompt, but i can
simply press <enter> and i get in.

is it my fault? can somebody confirm this behaviour on other
postgresql installations?
any hints or suggestions what i can try to fix this strange problem?

jan