What being “opt out” means is that the default is you are part of personalized search unless you clearly and explicitly opt out of it. In other words, the change here is that even if you are logged out of your Google account, you’ll still get personalized search based on an “anonymous” cookie assigned to you as a non-authenticated user.
Prior to this change, if you logged out, the act of doing so also opted you out of personalized search. Now you will need to take an additional step to do so. And I believe that opt-out still sends a tracking cookie to flag your preference to not receive personalized search results.
My concern here is that there has been quite a bit of disturbing research by Paul Ohm and others suggesting that anonymized data is anything but. If your so called anonymous data collected by Google through personalized search results touches any number of a small set of other data points, it may be possible for an interested party to correlate those touch points with the supposedly anonymous data and reveal personally identifying information, the toxic waste of online privacy.
Let’s face it, though, Google has all the incentive in the world to collect as much information about you as possible so this change is hardly surprising. They are in the business of selling the most effective advertising that they can. To drive better results for ad sales and hence more revenue by margin and volume, they need data, lots of it. The better they can correlate ad impressions to your paths you browse through the net and ideally the purchases you make along the way, the more compelling their ad service.
My hope is that scrutiny will keep pushing them closer to better behavior around privacy despite the gravity distorting pressures on them towards collecting more data. Also, its possible we’ll see more third party hackery, like the Firefox TACO add-on, to give choice and protection back to web surfers.