Leonard0 - 6:30 pm on May 29, 2010 (gmt 0)

We bid twice as much for those clicks..

So that means that Google's (and our) revenue could increase as a result of showing IBAs, assuming that the CTR of IBAs and context-based ads (CBAs) are equal.

Could it be that the clicking history of each individual user plays a role in the Adsense algo?
Each CBA's CPM could be adjusted by the user's CBA clicking factor and compared to an IBA's EPC adjusted by the user's IBA clicking factor.
(Sorry about the alphabet soup.)
Someone such as Martinibuster with a history of few or no clicks on CBAs would be more likely to be shown IBAs, especially after clicking on one.
The user's IBA clicking factor should be reduced as time progresses from the ad's first appearance or from the user's last click on an IBA, but that seems not to be happening in some people's experience.