You can use it to find usernames and passwords, confidential code, buffer overflows, and all sorts of other things. (Another news story here.)
Obviously, this isn't Google's fault -- if you put code on the internet, expect people to find it. Yes, it's a lot easier to find now with this tool from Google. As one commenter in Schneier's blog points out: "Essentially, this will force a massive audit of existing internet code." The list at kottke.org is an interesting list of things that people are finding with this tool.