Hi!
Some of this has been implemented via ideas like mysql-cluster
(mentioned earlier) but I gather that mediawiki itself would need to be modified to support that technology.
MySQL Cluster is designed to run in low-latency environment. Though local gigabit ethernet provides quite good round trip time, specialized server interconnects (like dolphin one) help even more. Geo-balancing on cluster is pain (unless you have really good interconnect, sub-10ms). And you may want to run one of late 5.1 snapshots.. ;-)
Load-balancing a webserver is already something that's fairly well known.
We do lots of database load balancing, with replication. It still has single master for write requests, though.
intelligently balance the load over to those mirrors which have bandwidth left.
Just run a caching proxy.. ;) It is probably possible to implement that as PHP script too, though it would suck.
This is a complex issue.. and probably not worth the spare change
which basic hosting would cost. If the site is wildly popular.. then become self-supporting via memberships, ads, donations, etc..
Dedicated servers nowadays are at 50eur/month range. With unlimited traffic \o/
-- Domas http://dammit.lt/