We maintain several DS3 connections to separate providers and make sure we are always online. Our infrastructure is protected from outside attack by several firewall, and intrusion detection tools, combined with security conscious and experienced staff. Your site will be hosted by several separate web servers. Layer 4 switching load balances incoming requests, and routes around defective servers. If all but one server were to fail, your site is still online.
All critical components have at least one hot-swappable backup.