Nitesh Dhanjani, gives some good examples on how you can use google to search for security vulnerabilities in your website.

He shows how to use the different searching strategies provided by google to hone in quite easily on vulnerabilities like indexable admin directories and exposed vnc servers.

The technique is a usefull addition to a site designers armoury to ensure that they have not left open any easily exploitable vulnerabilities - you can be sure that all the hackers out there will be using it as a way of finding sites to deface!