I had to update some customers' machines running official RHEL5 to RHEL5.1 ... i've just discovered that the reposync tool (included in the yum-utils package) is now included (CentOS had it already included even for the 4.x branch ...).
That means that with just reposync and createrepo (also available by default on rhel5) you can create very quickly an internal 'updates' repo without having to use :
either the RedHat solution to have a commercial version of rhn/proxy satellite inside of your network
either use mrepo (even if mrepo can do more things that reposync ...)
Now the underlying question for the CentOS team will be : 'which version will we ship in extras : our previously rpm packaged version or the one released upstream ?' ...