Password authentication failing...?

Hi there!

I've just installed Postgres and now I'm having problem with authentication.
Everything works properly when I log on "directly" (logged on to UNIX as
the same user as I'm logging onto Postgres):

[postgres@urd pgsql]$ psql ichallenge
Welcome to the POSTGRESQL interactive sql monitor:
  Please read the file COPYRIGHT for copyright terms of POSTGRESQL

However, as soon as I try to use the "-u" option to specify a username
and password, it fails, no matter who I'm logged on as:

[postgres@urd pgsql]$ psql -h localhost -u ichallenge
Username: postgres
Password:

Connection to database 'ichallenge' failed.
PQexec() -- Request was sent to backend, but backend closed the channel before r
esponding.  This probably means the backend terminated abnormally before or whil
e processing the request.

I get the same results no matter who I'm logged into UNIX as.  Once, I
was lucky enough to get a different failure message, saying that plaintext
password authentication had failed, but I can't duplicate it anymore.

Can anyone help?

 - Drew