There's a tutorial over on mpwebwizard.com that's been updated and talks about using a mirror to handle hits from a popular website (like digg or slashdot).
caching services such as coral content distribution network provide an alternative for when your site goes down, but why wait until it gets to that stage? why not just send all your hits from high-end websites to the coral cache version? well, there isn't a good reason not to, so i'll show you how to do it automatically.
He provides the simple script that looks for certain sites in the "HTTP_REFERER" value of incoming calls and pushes them off to the cached location with a header() redirect. The whole thing is only 12 lines, not counting the $sites array.