To all the Adobe AIR developers (both iOS and Android), here's a FREE and always updated Admob Native Extension (ANE) by Code Alchemy.
In fact, I have a post previously that talks about how to import the Admob ANE into the Flash Builder 4.7 properly. But the thing is, this time, at the time of writing, the Open Source Codes of the Admob ANE provided by the Code Alchemy has indeed included everything you need - Sample Project + Native Source Code! Pretty Cool huh?
Basically, you just have to import the Sample Project and start getting your hands wet in testing out all the available features. There is one impressive thing to mention here - the author has made everything so intuitive! Change Banner position, Banner Type, Interstitial Ads and etc. Customize the Admob to suit your app!
Apart from the full-fledged Adobe ANE that works perfectly both in iOS and Android, the author is always available for feedback and future enhancement. Drop him a message and he will respond with useful answers and help.
I wish there are more generous people like him to make our lives easier, as an Adobe AIR app developer. :)