On most devices, using AdMob in an obfuscated application (using ProGuard) works perfectly fine. However, on a few devices (I don't know which ones), you'll get a java.lang.NoSuchMethodError and your users will be angry. Here's the full stack trace:
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:200)
at