I figured I'd post this for anyone else who had my problem and runs into this thread like I did.
I had problems with installing courier on centos 5 as well, but the reason was different:
In the centos release file (/etc/redhat-release) it said release 5 ... and the script tried using /centos/5/ in the urls to get the files... these are 404s (I really recommend that cpanel people add a symlink 5->5.0). I changed it to 5.0 in /etc/redhat-release, and then it worked fine.