I have noticed through experimentation that certain ad block sizes are displaying ZERO Ads in many niches... and I'm not talking small niches either. I created a test page and loaded up all the sizes than posted and tested to see what would display.
On another and interesting note... About the time Google purchased Doubleclick Yahoo Sucked up Right Media.
What is interesting is this post
http://searchengineland.com/harvard-...ht-media-16154 about Yahoos deceptive advertising practices. This is not the only post on the subject, several sites have been talking about it over the past couple of months.
Just another example of the Big Boys telling you what you can and can't do in their TOS then doing whatever they want to do. Rules always apply to the other guys and Corporate Ethics is an OxyMoron.
Don't look for it to change anytime soon. Between Google, Yahoo, and MSN the inventory of Display Media Companies is owned. Competition is only between the Major Players and Yahoo is willing to do anything to grow their share of the Display Advertising pie... even if it means being completely deceptive.