There are systems that do something similar, particularly for Example 2. What Would Seth Godin Do, for example, stores a cookie in the user’s browser that tracks the number of times they’ve visited the site. If they’re a new visitor (have visited less than 5 times), it displays a message suggesting they subscribe to the site’s RSS feed.
It would be a simple matter to build a similar system that displays a Google ad for first-time or new visitors but not for returning visitors.