In a move that was somewhat anticipated, AdMob has launched an iPhone exchange for developers seeking to drive more downloads. Apps/developers contribute a portion of their inventory to the exchange and they get ad space on other apps.
It creates various opportunities for apps to be discovered by users of . . . other apps. This is a logical extension of what AdMob was already doing in advertising apps on other apps.
The growth of this apps "ecosystem" benefits AdMob in the long run as it becomes the ad network that many of these developers will use to monetize their free apps. AdMob also needs to continue to promote downloads because many apps are "perishable" and don't have a long iPhone shelf life.