security question

Hi,

I have a quick question about my installation that puzzles me. pgsql
8.2.3on a debian box. Tested it out the database works. pgadmin III
connects fine
too. I used alter table on the template1 database to give postgres user a
password. I changed pg_hba to use md5 on all connections including local.

When I use psql -U postgres -d myDataBase to connect to myDataBase I do not
get prompted for a password? When I su postgres and use psql myDatabase I
get prompted for my password?

Is there some reason why I can login using psql to a database when postgres
is supposed to use passwords for all access?

Probably a mistake I made somewhere along the line... just not sure where,
usually I get my pgsql install working great!

Thanks

Dave