The question itself is very good. I`m waiting for this...it`s gonna happen. It seems to be ok, although I think nobody tested it yet.
I see this is being asked the other way around in the Yahoo forum. Google and Yahoo both ignore each others ads when they calculate SERPs. Penalizing each other could be considered anti-competitive behavior, which could lead to stiff fines.
No. But i would present a different reason than above mentioned by friends. As yahoo display ads using javascript so they have nothing to do with links juice passing so Google doesn't care about them. And in result no effect on SERPs.