On the Outs with Ad Inventory

Today's Online Media Daily squib on the Hillary Clinton blog ad flap caught my attention not because of the tempest in a teacup raised over which blogs her campaign should or should not be advertising in, but because of something else entirely.

After the podcast series ended Wednesday, her campaign requested that the right-wing blogs remove the ads. The campaign promised to continue paying for the ad space--in effect paying the blogs to not run the ads.

Did other ads run in their stead, in effect allowing the sites to double book that inventory? Or did no ad run? The former is intriguing because it clearly increases one's ad revenue. But the latter is even more intriguing -- are there any actual instances of defensive ad buys? Reserving ad space and not using it with the intent that another advertiser couldn't use it?