From Nate Anderson’s “Vint Cerf: one quarter of all computers part of a botnet” (Ars Technica: 25 January 2007):
The BBC’s Tim Weber, who was in the audience of an Internet panel featuring Vint Cerf, Michael Dell, John Markoff of the New York Times, and Jon Zittrain of Oxford, came away most impressed by the botnet statistics. Cerf told his listeners that approximately 600 million computers are connected to the Internet, and that 150 million of them might be participants in a botnet—nearly all of them unwilling victims. Weber remarks that “in most cases the owners of these computers have not the slightest idea what their little beige friend in the study is up to.”
In September 2006, security research firm Arbor Networks announced that it was now seeing botnet-based denial of service attacks capable of generating an astonishing 10-20Gbps of junk data. The company notes that when major attacks of this sort began, ISPs often do exactly what the attacker wants them to do: take the target site offline.