Google's homepage is sacred. Despite being an ad company, Google has never run an ad on its homepage and likely never will. Ads for third-parties that is, it's more than happy to run ads for its own products and services there. It doesn't abuse this, which is why, when it does run an ad, people notice.
And people noticed an ad for the Nexus 7 greeting them on the homepage. Whereas Google was happy to just put a link there, the new ad is a lot more visible and animated.
Only US users got to see the ad, which makes sense as Google has only now started making available the device outside the US, with France, Germany and Spain being the first.
The tablet also sold like hotcakes so far, so Google is probably only now keeping enough of them in stock to warrant an ad like this.