I get anywhere from 1%-3% clickthrough, depending on the app, with AdMob. Whether your ads blend in with your app and where you place your ads affects this rate. AdMob recommends that you place your ads at the end of a user action, as in, once they've played a few rounds of a game, present them with their score and an ad. I think that's exactly what Angry Birds does.
I ask because I have trouble believing that people click on the ads out of intention. I think I clicked on one by mistake before (although, honestly can't remember for sure).
I just find it hard to believe that Angry Birds makes $1M in ad revenue per month. I don't get why you'd click an ad when you're about to play a game.