hi, as 5.4 was released and it's include support for kvm (what's more it's working for us since a few days) i plan to remove all of my packages form my repos as centos 5.4 will be released: http://www.lfarkas.org/linux/packages/centos/5/*/virt/
there are few trick although eg in the xml conf files you have to replace /usr/bin/qemu-kvm to /usr/libexec/qemu-kvm. and it's better it you first remove all of your current kvm related packages install the new kernel, reboot, remove the old kernel and only after intall the new packages (otherwise you've got a lot's off error and warning about weak-updates)..
ps. we finish rhel 5.4 rebuild at the weekend:-) there are a few tricks, some of them have to modify to rebuild like etherboot or build with special option like zsh, kvm, *kmod etc.