A short WebmasterWorld thread has a post from a member that I know, knows his stuff. In short, Google’s display ads, the images ads you see typically through the AdSense product on content sites, have a much higher mouse over rate, when compared to the click through rate (CTR).

This advertiser said:

The advertiser wants clickable mouseover actions to help increase the click through rate. I doubt that will happen, but it is interesting to know that people are hovering their mouse over the ads and not often clicking.

Of course this is just one case.

Forum discussion at WebmasterWorld.