The whole point of having an ad server is to make sure the right ad is served to the right person. You can serve desktop ads to desktop browsers and mobile ads to mobile in DFP. Or, you can book a mobile version into a mobile placement and a desktop one into a desktop placement.
Either way, this is something that can easily be handled on the server with existing tech. I don't see much practical use of his example.
The goal with responsive design is to serve the same site—the same mark-up, the same assets—to all devices. Which means that any conditional serving of ads has to happen client-side.
Do you see publishers getting on the bandwagon with serving the same assets across all platforms? The ones we work with have already sunk thousands and thousands into separate rigs for each place they run content.
Just wrote you back! You know, I actually don't see major publishers embracing responsive design, at least in the short term. I think mobile-specific sites and dedicated apps are much more inline with how they see the world. It's also a business opportunity for them to sell separate experiences as separate products, right? Merging everything into one isn't necessarily a win.