When a cron job gets stuck on OS X, it sends an email to the root account to let it know there's a problem. Unfortunately, you never normally get to see this email as it's routed straight to /dev/null. Here's how to get the emails sent to your own email address (works for me in 10.3.4):
sudo pico /var/root/.forward
change /dev/null to your email address
(this tells the system which address mail for root should be forwarded to)
sudo chown -R postfix /private/var/spool/postfix
sudo chown root /private/var/spool/postfix
sudo postfix start
(this sets some file & folder permissions then starts Postfix, the mail transfer agent)
sudo pico /etc/hostconfig
change MAILSERVER to -YES-
(this makes postfix start up after a reboot)
then to test everything, if you like:
mail root
Subject: test
. (period) then Enter
(wait a while after sending this mail, it can take quite a long time to be sent)