a little disturbingly, it looks like admob/adview continues to eat up resources for a minute or two after the
app has already gone into the background, even if one programmatically removes and destroys the adview
instance just prior to going into the background..
once this following webviewglue message appears in the logs, for the app, the cpu usage *finally* goes to 0.0.

  webviewglue   nativeDestroy view: 0x...