A lovely 'one liner' for printing out all the packages from a particular repo in Yum. Very handy if you want to uninstall all from one to reinstall from another.
rpm -qa --qf "%{NAME} %{VENDOR} \n" | grep "<VENDOR_NAME_HERE>" | cut -d ' ' -f 1 | sort
Taken from this post on one of the CentOS mailing lists.