Google search results are NOT case sensitive. All letters, regardless of how you type them, will be understood as lower case. However, there are certain words called capitonym whose meanings change when capitalized, for example, March (month) & march (walk).

Now there is a Google App Engine based third party application that will search queries in Google, scan through the output and filter the results based on the case of letters. But there are some limitations- phrase search (quotes), negative terms or any other advanced search options are not supported.

Click here to visit the Case sensitive Google search web application.