problems with permissions

All,

Having a problem dumping a database using pg_dump. This is Mac OS 10.9.2 and postgres 8.3.  I'm using md5 for
authentication,and have the appropriate .pgpass file with u=rw permissions in the user's home directory.  I can access
mydatabase, called radiovision, using psql -U radiovision.  I think that means the authentication is working properly
(i.e.postgres does not ask for a password for radiovision or give me any authentication errors).  However, when I try
tocreate a backup using pg_dump -C radiovision, I am prompted for a password to which I respond with radiovision's
password. I then get the error: 

connection to database "radiovision" failed:  FATAL:  password authentication failed for user "DrRudner".


Why is postgres asking for a password for DrRudner?  This is the mac username of the user currently logged in, and also
theuser that installed postgres initially. 

Any help is much appreciated!!

Regards,
Eric