DaStarBuG - 11:04 pm on Jun 23, 2011 (gmt 0)

That is re-targeting. If you were interested once, chances are you will be again. And stats prove that over and over. Otherwise advertising networks and for sure AdSense would not do it. They have hundreds of millions of clicks to prove that it works. By the way, IBA Ads still take part in the auction so they pay you more if they are shown.