This is the sort of thing that seems hugely distracting for a day or two, and in a week or two you'll be so used to it that, if someone were to take it away, going back to the way it was would seem like a huge distraction. For about two days.
Obviously, the nav bar has been internal for a while. I was not a big fan when I first saw it either, but now I've grown to quite like it. It's across all the products, and it sort of stamps the page as "you're at Google", much in the same way that you feel when you see the orange at the top of HN, or the blue at the top of Facebook. Even if you're tuned out, its in your peripheral vision and you get that sense of place.
That is true, eventually our brains will readjust and we will tune it out as usual. I suppose for the meantime the huge contrast will draw more attention to Google+ when they release its full features for everyone.
