As a matter of policy and because I'm at the end of a DS/1 that needs to feed my customers needs, I've set up a local subset mirror of Centos 3.3. It's a subset because I don't copy any of the x86_64 directories ( no machines that use them ) but it's also a superset because I've added an additional "local" directory structure that contains things that I need - amavis, clamav etc. I can do "yum install amavisd-new" from any of my servers and have yum install or update the packages that are needed.
The advantage for everyone in doing this is that I don't have to use up my DS/1 bandwidth for installs or updates of multiple machines and the mirrors don't get hit with a dozen requests for the same file from one location. (I was quite annoyed when the updated yum package wanked up my custom yum.conf) An additional benefit to me is that updates to my real customer servers go at a hundred megabit rate rather than the time it'd take to suck down a new kernel or openoffice through a DS/1.
I'd like to recommend that everyone with some kind of server farm do this. The local mirror doesn't have to be wonderful - mine is a 200mhz PPro machine built out of junked parts - all it has to do is serve FTP. You'll save money in the long run and reduce the cost to the donated mirrors. Someone pays for the bandwidth, one way or the other.