Zombies inevitably turning into botnets represent a huge, automated and efficient advantage to malicious attackers, I topic and most of its dimensions I covered in my Future trends of malware research. CipherTrust's Zombie Stats help you measure the approximate population of infected zombie PCs according to the vendor's TrustedSource. Not surprisingly, China's steadily increasing novice Internet population, both represents a growing menace to the entire Internet, and a market development opportunity for AV and security vendors. The situation is getting of hand with ISPs upgrading Internet connections, while still not putting enough efforts when it comes to dealing with botnets. And while some are taking actions under enforcement, major ISPs are still reluctant to face the issue -- ISPs still prefer to offer security services on a license basis or through reseller partnerships, though I'm certain there's an entire market segment waiting to be discovered by them if they manage to reset their position in this space.
Moreover, Prolexic's Zombie report for Q1-Q2 2005, provides even more detailed info, and a neat visualization of the routes involved with DDoS attacks, where the blue represents the U.S, and the red China. For the the time being, the ShadowServer guys keep on enthusiastically dealing with the problem, for no profit at all.