I literally just got off the phone with AdWhirl, getting a briefing on their iPhone app-ads aggregation offering and a colleague showed me this article about AdMob severing ties with AdWhirl. AdMob VP Ali Diab's blog post explains the rationale:

What we have also found over the last few months is that mediation layers significantly impair AdMob’s ability to optimize the selection of ads for the apps that choose to use them, by obstructing our view of these applications’ traffic profiles. These traffic profiles are a key input parameter that we use at AdMob to select the right ad for the right app at the right time.  By working directly with the publisher, AdMob is able to generate a much more accurate profile of the traffic, in terms of both volume and timing, generated by a specific iPhone app, which will enable us to optimize the revenue potential that we can deliver for the app.

AdMob characterizes companies like AdWhirl as a "mediation layer." Another way to describe them would be as an "aggregator" or even "exchange" (though that word has almost no meaning anymore). AdWhirl describes itself as an "open platform" that provides publisher-developers of iPhone (and soon Android) apps with more control, higher CPMs and fill rates by drawing upon multiple mobile ad networks including: Quattro Wireless, Videoegg, Jumptap and Millennial Media (Google AdSense soon). Until the AdMob decision it was part of that group as well. 

The company is a little over two months old, just raised $1 million and already says it works with more than 1,000 developers of iPhone apps. It also says it's serving ads against more than a billion page views already. 

AdWhirl's position is that AdMob wants to retain control over the developer relationship, while AdMob cites quality and optimization as reasons that it is no longer working with AdWhirl directly. But this is a microcosm of the larger tension that will continue to exist between individual networks and those that roll them up or seek to provide layers of control on top of them. 

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Related: Greystripe said it would offer a "CPM protection program," guaranteeing higher ad rates to iPhone developers for a limited time (via MediaPost).