Google has written a blog post documenting their legal process and approach to dealing with government requests for user and search data, highlighting a new section now available in Google that also answers more of these questions.
Governments routinely ask search engines like Google for access to user data for various reasons. From search logs, to email accounts, to browser history, to purchase history. Google says they take each request incredibly seriously.
They post transparency reports for all user data requests issued by governments around the world. In the U.S. they even document if the requests are subpoenas, court orders and/or warrants.
For more info: http://goo.gl/XkutF