According to a Massey University study:

“…hacking” attacks are on the rise…Dr Ellen Rose, a senior lecturer at the Institute of Information and Mathematical Sciences, and graduate student Natalia Nehring, ran Google search queries known to return sensitive information from the Google database. They wrote a computer program that for three months ran 170 daily queries against the Google database, looking at sites in New Zealand, Australia, the United States and the Czech Republic”.

The results weren’t too good, especially outside the US.

To check if your websites are vulnerable, the “Google Hacking Database” lists a lot of potential problems.