For a lot of people, Google is "the internet". So it's perhaps jarring to head to Google.com and see a giant black rectangle erupting from the bottom of the screen, eventually revealing itself to be an ad for the Nexus 7, Google's $200 tablet. Google... can't... have ads and sell things on its homepage, can it?
It can, and has, going back to the very first Android phone in 2008.
The first Android phone was (I think) the very first ad on Google.com, with a simple text link.
Google did it again one year later for Motorola's Droid, which launched Android 2.0.
With the Nexus One, the first phone that Google built in collaboration with a handset maker, it even got a tiny picture. Update: And let's not forget its Chrome browser and Chromebooks:
Google today is fundamentally different than the Google of 2007 (and 2005): Google's primary product for end users is no longer search. Or the internet. It's Google, as Mat Honan laid out here — the most visible symbol of which is the black strip that's now always there, hovering, reminding you of Google+, the clearest manifestation of Google as Google's primary product, particularly now that it's worked its way into more Google products than most humans can count.
So an ad for a Google product that's more visible and more visually disruptive than ever, literally bursting through the empty space that had been the trademark of the old Google, shouldn't surprise you. It's just Google selling Google.