Since F-8 yum repo definitions used "failovermethod=priority" by default. This makes it possible to force clients on your network to prefer your local mirror first before trying random other mirrors. mock should use this option for all /etc/mock/*.cfg files that point at a mirrorlist.
Changing version to '9' as part of upcoming Fedora 9 GA. More information and reason for this action is here: http://fedoraproject.org/wiki/BugZappers/HouseKeeping
This has been done.