Not sayign that he is a fool, but Snape must be very good, because he's definately fooling someone...
Yeah, so far, Snape's managed to become quite important for the two most powerful wizards of the world, and is fooling (at least) one of them-there must be something with that man!
Speaking of trust, Neo- do we actually know DD trusts him? Snape's apparently one if his highest ranking spies in Voldie's rows, if not the highest of all. And when he changed sides, Volemort was at the climax of his power. Can't it be that DD doesn't fully trust him but had no chance? They needed people like Snape, that's for sure! Maybe DD only gave him the position of the Potions Master to keep an eye on him....