Empath,
I think much of the difficulty with mirror sites is the syncing. For where it is used most commonly , FTP, site content is not updated as frequently as a message board. Changes made to the primary site are echoed to their mirrors and automated updates are typically scheduled such that it doesn't occur every minute.
For a highly dynamic message board such as the CPF, syncing may actually increase traffic as each "mirror" tries to update the rest to stay in sync overall. If it doesn't update frequently enough, the mirror servers will drift in sync and have yet more to update each other with eventually.
It's technically not difficult (i'd use rsync for linux) to achieve synchronization, but there are other considerations, like unequal bandwidth supply, security and administration (like keeping the servers updated with the latest software).
IMHO, a distributed server cluster architecture will work better, but that may require a little more co-ordination and cost, bordering on the highly professional.
I may be a little behind in state-of-the-art network synchronization / database replication, but not that far off the truth about mirroring.