Google have made some big changes recently to the SDK for Android; it's now included as part of Google Play services 4.0.
If you're an Android developer you'll be familiar with Google Play services, a unified platform which makes it easy to integrate Google features into your Android apps, delivered through the Play Store and updated at regular intervals. Now that AdMob is part of the package, a key benefit is that changes to the Google Mobile Ads SDK for Android get pushed seamlessly to users through Google Play services. For most SDK updates you don’t need to update your apps each time it changes, saving you development time. Follow these instructions to install the SDK.