When Google launched personalized search, it was for signed-in users only (and then only when Web History was enabled for the account). Now, guests or signed-out users at Google will get personalization.
If you don't want the personalized experience - you can opt out.
Google uses an anonymous cookie to keep track of signed-out searches in order to provide the personalization. When customized search results are available, you'll see a link for "View customizations" in the top right of the results.
You can read Google's complete announcement here.
What do you think of Personalized Search being made default for guest users? Let us know by leaving a comment.