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.