If you use a kickstart file to build unattended servers (eg virt-install) then new disks or virtual volumes will cause problems unless you have "zerombr true" in the kickstart file eg zerombr yes clearpart --all --initlabel part /boot --fstype=ext4 --asprimary --size=100 part swap --asprimary --size=512 part / --fstype=ext4 --asprimary --grow --size=1

(CentOS 6.2 didn't need zerombr).

Also FWIW, lines such as repo --name="CentOS" --baseurl=http://repo/CentOS/DVD/CentOS-6 repo --name="Updates" --baseurl=http://repo/CentOS/updates/centos6/x86_64

now work. In CentOS6.2 this would fail with dependency problems. Whether that's just 'cos the updates repo doesn't have conflicting packages so far and it'll break in the future, but it's working today :-)