Question: I have a responsive design I am working out that starts around 960 pixels wide and reduces down to 320px using css3 media queries.

I'm currently using a 728px banner above and below my content, but this obviously breaks once you get below 728px. 

What I need is the easiest way to display a smaller ad once the browser width goes below 728px. I can't simply load the ad units in divs and hide them using css because this is against adsense TOS.

Any suggestions?

Answer: 
1. Use a responsive AdSense unit.
2. If this is not what you want, use JavaScript to detect the available width then select the appropriate ad unit, then inject it to your content.
It is possible via hidden div and media libraries but I think It's forbidden by google :(