UPDATE 10/18/2010: Since writing this post, I have upgraded to CentOS 5.5 and have switched to using OpenDKIM to sign outgoing mail. It will also work great with CentOS 5.4. An updated blog post with step-by-step instructions is here.
I found this post from darkness very helpful when I moved from an older RHEL to CentOS 5.4 on a...