I migrated a centos 3.4 machine to 4.0 (very very interesting process, not for beginners!!! udev screws with things, has to be running 2.6 based rpm kernel etc in order for it to work because udev requires sysfs anyways yeah....). Its running cpanel flawless. no problems with exim. It had to build exim from the source rpm but other than that its working great.