You people with websites getting like 50,000,000 impressions per month, they can have highly-customizable ad units if Google lets them be a premium publisher.
may be you get some information here: http://forums.digitalpoint.com/showpost.php?p=7293740&postcount=5 http://forums.digitalpoint.com/showpost.php?p=83706&postcount=2
I also wanna know... I have seen Adsense ads some where (ezinearticles.com) where the have used hover and alt effect on google ads.... HOW TO ACHIEVE IT