Doubleclick doesn't allow proxied ad distribution, and I'm not sure a similar effect could be achieved through DNS. The Ars thread touches on this, where Kurt

Doubleclick doesn't allow proxied ad distribution, and I'm not sure a similar effect could be achieved through DNS. The Ars thread touches on this, where Kurt mentions that Doubleclick's ad deployment isn't smart enough to detect flash-blockers and serve a static ad, and Ars isn't permitted to replace Doubleclick's deployment code.