I’ve noticed a few sites showing altered Adsense blocks, using bigger fonts, bold text, and a variety of fonts. Given this level of flexibility, it is easier to blend Adsense with site copy. Scroll down through this site and you should see some examples.
I wondered if there had been a change in policy, so asked Google about it.
They replied “the feature you’re referring to is one of several new features the Google AdSense team is currently testing for a very limited number of publishers”.