It's rare to get a definitive confirmation from Google or its partners, but the revenue split was reportedly one of the early concessions Google made to bring carriers into the Open Handset Alliance.
This article talks about the current revenue sharing scheme and how Google is trying to re-negotiate a bigger cut of the 30% transaction fee on app sales:
"70% of every sale goes to the developer, 25% goes to the carrier, and 5% goes to Google. The new deal would cut the carrier’s share to 15%, which is what Google would also get."
http://www.androidauthority.com/google-share-play-store-andr...
This one has a statement from Google about how they share mobile ad revenue from searches with carriers but qualifies that the revenue sharing doesn't include apps like Youtube and Maps (interestingly it doesn't mention whether that's the case with mobile advertising on 3rd party apps):
"Google (NSDQ: GOOG) provided a statement today, saying that it is sharing advertising revenues with carrier and handset partners, but clarified that it is limited to search and does not extend to other applications, like YouTube or Maps. They said it was a pretty standard deal, and works across all Android devices that have Google search."
http://paidcontent.org/2010/03/26/419-androids-secret-sauce-...
From the Q1 2012 analyst call, Nikesh Arora talking about OEMs and Carriers get revenue share from Google Play and Google Search:
"Yes. I mean, again, I’m not going to talk about specific of the TAC on mobile. But as you know, we get people using our devices both organically as well as through our distribution partnerships with carriers and OEMs. And typically, the OEMs and carriers participate in some of the economics that are on the Android marketplace or Google Play and some of them participate in the economics around Google Search just the way we would do syndication on the Web platform, which you do with many partners around the world. We have similar deals on the mobile front."
- Nikesh Arora, senior vice president and chief business officer at Google
http://www.asymco.com/2012/05/13/android-economics-an-introd...