Tina, we actually consider those impression pings (and all of the request/response traffice, really) part of the internal workings of the SDK, so we don't typically comment on them publicly. If you're using the SDK in accordance with our guidelines, your ad impressions will be counted correctly. Interstitials, for example, will automatically register an impression with AdMob when displayed.
In response to your question about banners, we don't count impressions for test ads. Because they're just for testing, there's no real need.
Thanks!
-Andrew