I've seen many sites with slightly edited adsense appearance. In which cases it is allowed? For example: http://ezinearticles.com/?cat=Business (different ad font) http://www.squidoo.com/birdrescue (smaller ad urls, "ads by google" not highlighted, logo not clickable.)
It is allowed in the cases where the Publisher is a Premium Publisher...which most of us are not. We mortals are not allowed to edit the code. Period.
Never modify the adsense code. Premium publishers like CNN, TechCrunch, Lifehacker, Mashable get millions of pageviews every month and hence Google offers them extra benefits. So, their adsense code could be little different.