I was going to post this in the updates forum but don't have thread access there - as I believe this will affect more then just Joomla. While this is Joomla specific, if you store site backups in your web root directory, you should read this.
From the Joomla site: "It has come to our attention that Google has released a new product, Google Code Search, that is capable of indexing and crawling through archive files stored in the public directories of web servers. We are reporting this as a security advisory because we have discovered that some site administrators are storing archives / backups of their website in the web root. Because of this, Google Code Search is able to crawl the archives and read unparsed PHP files as if they were plain text. This has resulted in the disclosure of some sensitive information including MySQL passwords and SMTP credentials."
The full article can be read at
Joomla.org
Bottom line - due to caching - if you have backups stored in your web root - You might consider changing your passwords for your site. Read the article and decide for yourself.