Automated backup problems - pg_dump

I am sure many of you have encountered the problems I am experiencing
with automating the pg_dump command from a cron. The problem arises from
a failure of this command to use the environmental variable,
PG_PASSWORD. (PGUSER, on the other hand, is picked up during command
execution.) The cron seems to fail completely when crontabbed as the
PGSQL superuser, postgres. I do, however, get partial execution when I
su to the user postgres from a cron crontabbed as root. Let me layout
some relevant info:

root orginated cron:

30 * * * * su - postgres --command="cd /var/lib/pgsql/skybuilders;
/var/lib/pgsql/skybuilders/dbbackup.pl; echo `date` > timeRun.txt"

skybuilders is a directory where I copy and dump PostgreSQL files and
DBs to be picked up by an rsync cron on an archiving machine.

dbbackup.pl is a perl script that generates a bash shell. I will excerpt
the pertinant (failing) section of the generated script
(dbbackupGenerated.sh):

PGUSER=someuser
export PGUSER
PG_PASSWORD=somepassword
export PG_PASSWORD
pg_dump -o somedb > /var/lib/pgsql/skybuilders/dbbackup/somedb.dump

When I run the script from the command line, after having su'ed into the
postgres account, this script stops at the pg_dump line waiting for a
password.

Finally I will excerpt some of the final lines of my pg_hba.conf file:

# TYPE     DATABASE    IP_ADDRESS    MASK               AUTH_TYPE
AUTH_ARGUMENT

#local      all                                          trust
#host       all         127.0.0.1     255.255.255.255    trust

# Using sockets credentials for improved security. Not available everywhere,
# but works on Linux, *BSD (and probably some others)

#local  all     ident   sameuser

### --> Added by JSB <--

local       skyTemplate                                  md5
local       sameuser                                     md5
local       all                                          md5  admins
# use following line to perform commands as postgres without password
challenge
# local       all                                          trust

host        all         10.10.40.25  255.255.255.255    md5

I understand were I to change the AUTH_TYPE setting from "md5" to
"trust" the password challenge emanating from the pg_dump command will
go away. However, I wish to maintain the use of md5 authentication.

Ideas anybody?

--

 Jesse Burkhardt
 jesse@skybuilders.com  (w) 617-876-5680
 goose@aerogoose.com    (h) 617-354-5523