A marketing department would pay the Agent for the views of his Ads as reported back from the devices. And I agree that the technical side is the easier part of the problem.
Unfortunately, the technical specification is the start of the process. If the wrong technical specs get out for advertising without the proper thinking done before hand, you could have the same problem happen to eBooks as has happened to HTML.
Right now, programming for the web is painful simply because the HTML standard grew too quickly and was adopted into practice poorly by the various Browser manufacturers. A little standardization at the first would have seen the web light years ahead of where it is now. Unfortunately now, a good web designer has to try to program to every type of device which views his pages.
The ePub standard is still in its infancy so NOW is the time to do the thinking. I see advertising coming whether we like it or not. Since it is coming, shouldn't the mechanics of how it could emerge be thought out so that the standard allows for its inclusion?
If we will have to deal with it, shouldn't we get our two cents worth in as to the proper way to do it?