I got this a couple of days ago. It links to: http://www.google.com/chromebook/index.html

Despite the panic in the Google+ comments, there's a cross next to the ad, just click it and you don't see it again.

It's no big deal, IMO.

Edit: this is the yellow box at the top of the screenshot on the + page. I'm use the current version of Chrome on OSX, in the UK. I have no plugins / extensions.