In August, Google released a field trial allowing Google search users to include their emails in their Google search results.
There was a lot of privacy backlash, so Google kept saying it was opt in and there are privacy controls. So Google waited for the steam to calm down a bit and now said on Google+ that this field trial is now open to more Google users.
Starting today, you can sign up for a new field trial that makes it easier to find your stuff across Google, whether you're searching on Google.com or searching in Gmail. In Gmail, as you begin to type into the search box, you'll instantly start seeing relevant emails from Gmail as well as results from Google Drive, Google Calendar, and more. Similarly, when you search on Google.com, your results will include relevant information and messages from Gmail and Google Drive.
Google Apps users are still not able to participate but everyone else, sign up over here.
Forum discussion at Google+.