We released a new version of the Ad SDK during //Build 2013 Greetings from BUILD! Announcing New Tools to Help Developers Boost Monetization in Windows 8
We have a new version for 8.0 and another for 8.1 that is now included in Visual Studio 2013. The new SDK can be obtained at http://adsinapps.microsoft.com/
In summary, the new version now enables targeting of Windows 8 and Windows 8.1 and supports changes in the available ad sizes.
To access this new Windows 8.1 Ad SDK, App builders can download the Microsoft Ad SDK for Windows 8.1 on our developer website: adsinapps.microsoft.com or take advantage of the Microsoft Ad SDK integration with Visual Studio 2013.