About six months ago said they prefer webmasters go with a responsive design technique for their web sites.
But like anything Google related, the paid side and organic side do not see eye to eye on things.
Google AdSense publishers for the past year or so have been pleading with Google to create ad units that work out of the box with responsive designs.
In short, more and more publishers are adopting a responsive design approach to their web design. But monetizing those designs are harder and harder. Amit Agarwal posted one solution six months ago but that may be considered against Google's terms of service, so you need to be careful.
The real solution is for Google AdSense to come out with an ad unit that supports dynamic sizing of ads along with your web design that supports it as well.
It is known that the Google AdSense team is seriously thinking (maybe working on) about how to achieve this and I wouldn't be surprised if we see something from Google on this front within the next 6 to 12 months. Hopefully sooner.
Are you frustrated with Google over this?