Network World: Who's got the biggest cloud in the tech universe? Google? Pretty big, but no. Amazon? Lots and lots of servers, but not even close. Microsoft? They're just getting started.
Household names all, but their capacity pales to that of the biggest cloud on the planet, the network of computers controlled by the Conficker computer worm. Conficker controls 6.4 million computer systems in 230 countries, more than 18 million CPUs and 28 terabits per second of bandwidth, said Rodney Joffe, senior vice president and senior technologist at the infrastructure services firm Neustar.
Here is a good paper on Cloud computing - information security briefing (PDF):