Postgresql allow connections without password

I have postgresql 8.4.9 installed now, my problem is that from time to time
my postgresql let psql version 8.4.9 access the database without asking for
password (psql -d mydatabase -h myhost -U myuser), and the connection
attempts from psql 8.3 are not allowed no matter what i got time out
exceptions from psql 8.3.
The first time that happen to my i have installed postgresql 8.3, so i
think that because i upgrade my system and not postgres (i upgrade from
debian lenny (5) to squeeze (6)) that was the problem, so i installed 8.4.9
and with pg_dumpall i restore all my databases. And the problem was solved.
Postgresql ask for passwords no matter from where i try to connect.
And now a month later i have the same problem with 8.4.9. After various
restart, stop and start the problem disappear, and 3 hours later it came
back.
I don't know what to do.
In my logs the only strange thing i startup package incomplete after i
start the server. I google it but nothing seems to explain it, and i don't
have any cron.

Now im going to install postgresql 9 to see if the problem disappear.

Any help will be really appreciate.