This is a very interesting article from Microsoft Research
that describes a method they use(d) to find web sites that exploit browser vulnerabilities, including zero-day exploits. It is called "Automated Web Patrol with Strider HoneyMonkeys
". This reminds me of the Windows Honeypots
book I read, as the researchers are cleverly using multiple virtual machine images with various patch levels plus known exploit signatures and other methods to find the exploits.