Ads are a necessary evil when it comes to keeping free apps free. Most developers are pretty cool about it and keep the ad in a low, out of the way place where you can effectively ignore it. Some developers, however, put the ad in a place where you are likely to accidentally launch it.
If that kind of tactic annoys you, you're not alone. Google expressly prohibits developers from using such tactics in both of their ad program policies. A developer has a better chance of making money by using honest ad tactics. If they sink to using such dishonest tactics, people are likely to stop using the app. One click out of 500 users is better than one click per disgruntled user.
You may be a developer just starting out and don't know any better. You might have placed the ad there because you thought it looked better in that position. You could have also just wanted to play the system because you totally knew better. Either way, Google has some suggestions to retain good will between yourselves, Google and the end user.
First things first, don't put your ad somewhere near the interactive elements of an app. If you're going to require the user to start touching buttons on your app, make sure the advertisement is far away from those elements. Let the user decide when they want to click the ad, don't decide that for them with shady app tactics.
In a related note, don't put ads anywhere near parts of the screen with heavy activity during games. Some games require the user to poke and prod the screen in a lot of different ways. Putting the ad in the way of those functions is only going to frustrate the user.
Google and the user understands that not all phones are created equal. Some phones are smaller than others and their screen sizes must be accommodated. If you find yourself developing for a smaller screen, make sure there is a visible border between the app and the ad. It helps no one if the ad is indistinguishable from the rest of the app.
If you need a good example that encompasses all the above suggestions into one, look no further than the piano example in the blog post:
Here you have some bad placement with the ad resting underneath the piano keys. A user is obviously going to start clicking the ad because they're too busy rocking out to notice where they're tapping.
It's a much better idea to put the ad above the app. Sure, it might not be as noticeable, but at least you aren't enraging the user by interrupting their impromptu iPhone piano rock session.
So now you see that good ad placement boils down to one simple rule - don't be a jerk. Treat your users with respect and they'll treat you and your app with the respect it deserves.