Penal substitution and forgiveness

This is a harder question philosophically I think. It's one that appears in the Lost Message of Jesus, and crops up again and again when debating with non-evangelicals online.

It goes something like this: Jesus taught that we should forgive our enemies without asking for anything back. Yet the heart of penal substitution says that God demands payment for sin - he won't forgive without it being made up to him! What's that about?

Well a number of answers:
First, I think it would be a mistake to say that we should expect God to give us commands which also apply to himself. In fact, there are plenty of things that God requires of us as creatures which do not apply to him as creator: not to worship ourselves, for example. When I demand payment or recognition or vindication for being wronged, that is inherently sinful, because, as a creature, I don't deserve or merit vindication. God, being God, does; the ultimate end of the universe is the vindication (or glory) of God.
But that's not satisfactory by itself, because this is, in some sense a moral issue, and we don't want to say that God expects us to be morally superior to him. However, I think penal substitution does, if we view God as Trinity, model forgiveness better for us than any other atonement model. If all three members of the Trinity are equally God (which they are, I believe) then God as a whole is, in punishing Jesus in our place, taking upon himself the hurt and pain and alienation caused by our rebellion. Isn't that exactly what God asks us to do in forgiving others? I think that's significant, because other models without penal substitution don't in any real sense model God's self giving, self substituting love. Christus Victor says that the atonement is all about God defeating his enemies. Moral influence is about God winning our hearts, moving us to repentance. But only penal substitution (or those other models with penal substitution as their centre) show us how to forgive - take the consequences of the sin of others on yourself. It, as a model of the atonement, shows us exactly what Jesus meant by forgiveness more than any other model.
Third, the whole argument is based on a false premise. It would undermine God's forgiving nature if he was demanding reparation from us, but the point is that the reparation is paid within the Godhead so that the Godhead's attitude towards us is one of forgiveness. In that sense, to set retribution and forgiveness up against each other is to set up the very problem the cross solves.
So what about forgiveness then? Should I only forgive because Jesus has taken the punishment for the sin that someone has committed against me? (Or that God will punish it in the future?) Well, no, I should forgive because Jesus tells me to, and I know what it is to be forgiven. But the great thing about penal substitution is that it does two things for me when I am trying to forgive.
It stops me ever later coming back and thinking that the sin committed against me should be paid for. It the safety net against bitterness, because that sin IS paid for. No more coming back to it. I have no right to dig up something that Jesus took away and buried with him in his tomb.
It also helps me with the "right" sense of "wrong" we have when someone sins against us: that sin was wrong and deserved punishment, and should be pronounced as wrong. The cross as the ultimate act of self revelation tells my troubled soul "God knew that was wrong and has publically, cosmically, wholeheartedly, at great cost to himself pronounced it to be so. You need not seek out a way to prove that sin is sin, for God, in simultaneously pronouncing his own glory AND saving you, has pronounced that it was wrong. Be calm and let it go, for God has already passed that verdict and served the sentence."