You can, but you're asking for trouble. Make an example page and email Adsense Support to ask for permission. Even if they say Yes, it is a very real possibility that they can come back to you later on in the future and tell you you are in violation (i.e. what happened to this guy). When it comes to Adsense Support, ask 3 times, and you're liable to get 3 different answers.
Mmm... good question. I know for a fact that you aren't allowed to place Adsense ads in a floating div or anything that obscures the content of a page. Not certain if you can have something move over the ad though.
If I read him correctly, I think he means have an animated menu directly above the ads, thus drawing attention to the ads below...
I was thinking in making a floating menu that move over a skyscraper type ad in the left part of the site...
Not necessarily. Sounds lke a gray area. Ask Adsense Support for permission, and then save the email if they say yes, just in case...