I have just resolved an issue that I thought someone else might run into in the future, so I'm posting it to the list for future googlers.

Problem Symptoms:

• OTRS  cron jobs are not executing, as a result no remote e-mail fetching is occurring
• Manually running "Fetch Mail" from the admin interface functions properly
• Manually executing "su -s /bin/sh -c /opt/otrs/bin/otrs.PostMasterMailbox.pl otrs" as otrs user functions properly
• Adding "opt/otrs/bin/otrs.PostMasterMailbox.pl >>/dev/null" executes properly.
  
• /var/log/cron contains the following errors:
• crond[6551]: Permission denied
  crond[6551]: CRON (otrs) ERROR: failed to open PAM security session: Success
  crond[6551]: CRON (otrs) ERROR: cannot set security context
  

System:

• CentOS 5.x/RedHat 5.x
• OTRS RPM install
• selinux permissive
• crond is running
  

Solution:
The cron.log error indicates that the problem is located somewhere within the PAM stack, which is preventing crond  from launching a shell owned by the otrs user. The system in question is using /etc/security/access.conf to limit login access to the server. (Useful for limiting login access when the system uses some form of centralized authentication.) Other required PAM modules may also interfere. In this case adding a line to permit the otrs user login access via cron:

+ : otrs : cron crond

or less secure

+ : ALL : cron crond