I followed the instructions as shown on http://wiki.dreamhost.com/Run_PHP_from_cron exactly, from the very beginning - including, while logged in via the shell, creating a bin directory if you don't already have one, running curl to get the site-check perl script shown under "We're going to now install a little script" and editing the variables.
Basically, I guess cron really can't browse the web because all it's just a scheduler, and what you're really trying to do is visit get_mail.php.
But the perl script "site-check.pl" can.
So after you plug in your two variables, then you can configure the cron job to run the perl script, not try to browse to get_mail.php.
It doesn't say it, but it was as if those instructions were created SPECIFICALLY for situations like trying to run get_mail.php, where you need a web site to be visited to kick off a job, in the case of Postie, to force a mail check.
BTW, I configured my cron tab to only run the script once every 15 minutes, and even that is way too often. In the shared server environment, I didn't want to try running something once every minute when I really didn't need it. If I am sending pictures to the blog more often then once every 15 minutes, I need to find a new life.
We don't need no stinkin' signature....