Black swans don't matter
Joel Spolsky makes some interesting observations about six nines uptime and the problem of black swans (statistically improbable events) but he has, I think, missed the point. The internet was intended, by the people who invented it, to be constructed as a web. By web I do not mean HTML, I mean heavily redundant interconnection. His system failure was not due to a fluke. It was not due to incompetence. It was due to the fact that his system hinged on a SPF (single point of failure).
Joel is spot on when he observes that visits from black swans are inevitable. They certainly don't arrive on anyone's schedule. The key is multiple redundancy for every last part of the system. If any part represents a SPF, there is a black swan out there. It's looking for you, and it only has to get luck once.