It isn't, but I seem to recall it being mentioned either in the keynote or in the reporting after WWDC.

Update: https://developer.apple.com/news-publisher/

> Monetization is made simple with iAd, Apple’s advertising platform. Earn 100% of the revenue from ads you sell, and 70% when iAd sells ads for you. iAd provides campaign management, targeting and reporting capabilities that help drive your business.

So that's very fair, they don't take a cut if you sell the adverts yourself - otherwise they take a (industry standard?) 30% cut.