Know this old, but very good question and wish I had good info on subject few years back...
From time to time we have (school activity related) sites featured on TV networks. Since we operate on a very tight budget, "load balancing" is the solution. VPS boxes can be had pretty cheap these days and we just mirror/duplicate our content on 2-3 of them.
Look at this article and read about "round-robin".
More info about load testing can be found here.
When we first started trying to handle spikes, we simply had our content on 2-3 VPS boxes and placed their NS in registrar settings.