Google has released a beta version of the AdMob SDK which adds support for Windows Phone 8, so developers no longer have to rely on Microsoft Advertising for ads. But, while the SDK does have some good features, it is still lagging behind the Android and iOS versions of the SDK. The WP8 AdMob SDK supports creating an AdMob banner view from code, embedding an AdMob banner directly in an XAML file, showing full-screen interstitial ads, and registering for ad events such as succeeding or failing to receive an ad.
But, unlike the Android and iOS versions, AdMob for WP8 does not include ad network mediation, support for DoubleClick for publishers, searching ads for mobile apps support, and MRAID support. Google's delay in adding WP8 support to AdMob goes to show how little revenue the platform generates for Google; and, just as telling, AdMob still doesn't support Windows RT.