Here's a picture that illustrates the ad that I see, and the MultiFox extension:
Apparently you can hit the close button and it goes away for a while, but it comes back. It's pretty annoying and I'll probably write a Stylish script to get rid of it for good with a CSS tweak.
Anyway, you can see where it says my nick that if I click on that little button I can instantly switch identities. It switches over to an entire new set of cookies that match that identity. Best of all, it's totally automatic, when you log in on sites it automatically remembers so you can switch to something else. You can even be logged into multiple accounts to the same site using different tabs.
Just based on what you've said it seems that Google was inspired by the MultiFox extension. In fact I was able to find posts on message boards from a year ago where some people, familiar with MultiFox, were looking for a similar add-on for Chrome. Not that plagiarism really bugs me, I mean the whole industry runs on it. I'm just pointing it out, just like when you point out when Opera has some killer feature first.