Hari K Gottipati, a software engineer with Motorola Inc, qoutes in his blog, a technique to earn Google AdSense Page Impressions without displaying the Google AdSense ads. It says: Google AdSense makes use of iframe to display the ads and stops you from fetching the ads to the hidden frame, Google has no way to find out whether the iframe is hidden or not.
“This weekend I was working on some thing where I needed to hide iframe based on some action. I figured it out putting the iframe inside div tag and hiding the div tag is very easy. And then I realised that Google adsense uses iframe to display the ads and wanted to see how Google prevents me fectching the ads into a hidden iframe. There is no way(as of now) for Google to determine whether the iframe is hidden or not as I am using div wrapper around iframe to hide it.