I did some googling tonight and found some more information about ways to do some magic distributed caching.
An example for a proprietary product doing this is Cisco DistributedDirector (19000$). Anybody?
Ok, so here is the open source alternative: * Super Sparrow http://www.supersparrow.org/ Open Source, linux, and tested. It's running for example vergenet, you can see it in action at http://www.vergenet.net/vergenet/.
In combination with Linux Virtual Server& Heartbeat plus distributed squid 'mirrors' this looks like a nice way for future growth.
IMO this is nothing for the immediate future, but good to keep in mind and start playing with. Maybe it would also be possible to ask Horms (Simon Horman, http://www.vergenet.net/~horms/) for advice, he definetely is an expert in this field.
Have a nice new year!
Gabriel Wicke