began rolling out last week, is actually a new measure to improve your surfing privacy, and not the 'Google +1' social layer that we've been anticipating.
Basically, the bar elucidates the three 'states' in which you can use Google's services. First this is 'Unidentified,' which is where Google only knows your IP address, tracks you with a cookie, but doesn't know your name; then there's 'Psuedonymous,' which is how most of us currently use Google -- and finally there's 'Identified,' which is where your real life identity is assured.
Interestingly, Google also answers a question we've always wondered about: when you're logged out, you are effectively anonymous -- even if your IP address matches one of your Google accounts, your surfing habits are still kept separate.
The navigation bar, then, is simply a way of making sure you always know just how private (or public) your actions are. It's worth noting that the nav bar might still become a part of the upcoming '+1' social layer -- but for the time being, we can only guess at how they might be combined.