In 7.1.2 you can launch this command without any problems:
pg_dump -u -s database < file.txt
where file.txt contains 2 lines:
username
password
~
~
~
And you'll get the dump of the <database>.
In 7.1.3 this command work, the only incovenience is that pg_dump shows "User name: Password:"
but gets the password from file.
In 7.3.x the pg_dump accepts the password only from stdin. The user must enter his password.
It's a feature or a bug, because i can't see any logic in this behaviour.
I hope i'll receive an answer, or else i'll post this topic on developers list :)
Thanks.