I have been in telecom for quite some years now. Five nines is the holy grail: 99.999% availability. It is much easier to achieve for a phone company than on the net. Proof of it: Las month, Google had 99.999% in only one country: Brazil. The rest were all 99.990%-99.998%.
The meaning of five nines is three minutes of downtime a month. Difficult? Yes, but still a goal we should all aim for.