su not working with psql

I'm new to this list and new to PostgreSQL, so I hope this question
isn't off-base.

I'm running version 8.2 in Mac OS X 10.5.8. I installed it in a new
"postgres" user account. I can access it via PGAdmin and via a test
application I'm writing, both from my normal user account, "david". I
can also access it in the Terminal using psql when logged into the
postgres user account. But when I'm logged into my "david" account and
try to access the database via sql I get the following:

imac:~ david$ su - postgres
Password:
imac:~ postgres$ psql fcddb
psql: FATAL: role "david" does not exist
imac:~ postgres$ whoami
postgres

I wonder if anybody has any idea what's going wrong? The system
appears to correctly see me as postgres after using the su command, as
evidenced by the response to whoami. But somehow the PostgreSQL
process or psql is seeing me as "david". I have tested to see if
creating a role for david in the database solves this problem, and it
does. But I'm concerned that there's something fundamentally wrong
here that needs to be addressed.