Washingtonpost is running an amazing piece on the mechanics and economics of operating botnets:
Hackers are hijacking thousands of PCs to spy on users, shake down online businesses, steal identities and send millions of pieces of spam. If you think your computer is safe, think again
Also included is an interview of a botnet operator, known as 0x80 (X-eighty) :
In the six hours between crashing into bed and rolling out of it, the 21-year-old hacker has broken into nearly 2,000 personal computers around the globe. He slept while software he wrote scoured the Internet for vulnerable computers and infected them with viruses that turned them into slaves.
