I like that idea, but I think it's against policy (see here:
http://helpcenter.admob.com/content/policies-tips-developers-publishers).
The statement, "Ads should not be pre-cached, cached or stored in any
fashion. All ads should be requested dynamically and refreshed when
the page is refreshed."
I think if you re-use an ad, and the advertiser's budget has run out,
then what does AdMob do? Not pay you for the click?
Anyway, if using an AdMob ad from the AdMob cache is legitimate, I
would like to know. I assume that it isn't, though, unless they post
here otherwise.
I'm sure they use the cache for that reason, but if they display a
cached ad and the advertiser's budget is done, that's their problem,
right?
-Jim