In a quick google search I was able to find a bug entry for this
http://bugs.centos.org/view.php?id=3584
Might be something their about it, generally I deal with installs via PXE so I don't suffer this so much, when I am doing installs via CDs the servers we use have a boot menu, we default CD-Rom after the HDD in the boot priority and just use the hardware's boot menu option to manually boot the CD-Rom, for hardware without this option their can be some playing about.