A bot net consisting of over 4mil bots was taken down this week. The operation was conducted by the FBI with help from Estonian officials, who took 6 suspects into custudy in adition to shutting down around 100 botnet servers November 8th, 2011. The take down is the result of a 2 year investigation.
The infected computers had their DNS settings altered to point to foreign IP addresses, which redirected the computers to malicious websites without the user even being aware of it.
It’s pretty impressive how a botnet that big can be dismantled. You can learn more from the FBI’s official statement here.