Unable to Run pg_dump

I have been using pg_dump to produce automatic backups using cron for several years.

I recently changed to postgresql-9.2.1 and now I get the error:

=================
mkdir created /notraid/dbbk20130319 correctly
Password:
pg_dump: [archiver (db)] connection to database "coaxpubs" failed: fe_sendauth: no password supplied
Bad database backup 0
=================

I think the config is correct for an unattended backup. I have pg_hba.conf set as:

local mydb cobkup trust
local all all password

and the following is the command in the script run by cron:

/usr/pgsql-9.2.1/bin/pg_dump -f /notraid/$dirname/mydb.`date +%Y%m%d` -F custom -v -U cobkup mydb

A search of the archives doesn't turn up any changes between versions 8 and 9 of postgresql regarding password insertion.

I suspect this is going to be a misunderstanding on my part but can anyone help?

Regards,

John