I'm still talking about a benevolent God here, and I'll suppose you are too: if an evil thing is merely a good thing we can't understand as such, then we'd have to revise our moral system, because the sense that harming others is bad isn't justified anymore if happiness and suffering are one and the same in our God's eyes.
But if God was a higher being, although not morally inclined, or if there was no God at all, then the yin-yang theory would be interesting because it would apply inside our own understanding of the world to point out the limitations of our (moral, notably) perceptions. But it's rather about God's perception we are talking about, I think.
Actually, thinking about it, don't you feel that free will (as ability not to do the right thing) in the christian God's universe is ridiculous ? The only way you can use it is by performing evil deeds, yet these can only be done out of ignorance of what is right, impotence to do what is right, or just plain cruelty that'll probably lead you right to Hell.
(Sorry for the english mistakes I've probably made throughout the thread. Please point them to me if you've got time on your hands.)