Admob support was added in one of the early 108 beta builds. In fact, using Google's new and improved Admob Mediation, you can get virtually any ad network to work via AppGameKit (even in Tier 1) whether you are making iOS or Android apps via the available adapters.
I wrote a guide on doing so to include iAd support alongside Admob in Tier 1 for iOS, with hints for Android as well.
http://forum.thegamecreators.com/?m=forum_view&t=207370&b=41
The biggest limitation, and one I hope Paul addresses in V2, is that we are confined to 320px wide banners no matter what the service. Smart banners are more popular these days especially with iPads and tablets, in that they stretch across the full width of the device and are much more detailed.