This scalability strategy is brought to you by Erik Osterman:

My recommendations for anyone dealing with explosive growth on a limited budget with lots of cachable content (e.g. content capable of returning valid expiration headers) is employ a reverse proxy as mentioned in this article.

In the last week, we had a site get AP'd, triggering 100K unique visitors to a single IIS server in under 5 hours. It took out the IIS server. Placing a single squid infront of the server handled the entire onslaught with a max server load of 0.10 on a modest Intel IV 3Ghz.

It's trivial to implement for anyone interested...