With CentOS, if you have the DVD or ISO install image on your CentOS system, just disable the [base] repo in /etc/yum.repos.d/CentOS-Base.repo and enable [c5-media] repo in /etc/yum.repos.d/CentOS-Media.repo and point baseurl= to the right location where the ISO or DVD is mounted (baseurl=/media/PATH-TO-DVD).
This works out great as the CentOS [base] repo and the DVD have the identical contents.
However, with Fedora 7 and newer, this is not the case and things are a bit messy. The contents of the [fedora] repo in /etc/yum.repos.d/fedora.repo have Everything in Fedora since the Core split was dropped, which is a huge amount of packages than ever ship on the DVD.
So, how to use local [InstallMedia] first, while still searching the [fedora] repo, and still get [updates]?
Fedora 12 has been the best install in my time of using Redhat/Fedora (RH5.2, RH6.x, RH7.x, RH8.x, RH9, FC1, FC2, FC3, FC5, FC6, F9, F11, and now F12). I started early with Fedora 12 Beta, actually, and have installed the released F12 on a number of systems since.
My home MythDora 10.21 MythTV system was showing its age (based on Fedora 10, which is no longer supported). I decided it was time to upgrade to F12.