Came across this fun 'state of the industry' report published by desilva+phillips in 2010. I found this quote particularly interesting:

"However, while ad networks got buyers thinking more explicitly about audience targeting, they were insufficiently clear about whom the audience being offered really was and how it was going to be reached. What publishers failed to foresee was that ad networks would make available huge amounts of non-premium, remnant and nonguaranteed inventory. As a result, supply grossly outstripped demand, driving down CPM rates not only for inventory the ad networks were selling but also for the publishers’ premium, higher-margin inventory. From the publishers’ perspective, their online display-ad inventory has been devalued by ad networks and undervalued in the minds of buyers. Publishers, moreover, have discovered that performance and direct marketers prefer to buy audiences they believe can in part be addressed through remnant premium impressions, rather than pay far more to buy the same or similar premium inventory directly from the publisher. The result is a vicious cycle: premium display inventory becomes harder to sell, and more of it goes unsold and becomes available on the ad networks, putting further downward pressure on premium-inventory pricing." (desilva+phillips)

The entire landscape of display advertising seems to change every 18 months, and the last few years have been no different. What's different lately, however, is that the devaluing trend seems to be reversing itself. Both publishers and agencies are seeing less value in the intermediaries, and instead are using those tools themselves. The net result is more value everyone: publishers, brands, readers and less distractions between them (see here).