Lately I've been thinking about forgiveness and the penal-substitutionary explanation for Jesus' crucifixion (yes, even before AUFS brought it up, despite my delay in writing). I think, perhaps quite simply, that forgiveness itself is enough obstacle to oppose the idea of substitutionary punishment.
Forgiveness is, after all, an act of the will that leads to reconciliation. Suppose two people argue and hurt each other, when they forgive each other they are reconciled. No one needs to be punished. Furthermore, during Jesus' ministry there doesn't appear to be any obstacle to Jesus just simply forgiving people. He said it to the lame man in Matthew 9.
Now, you might argue that Jesus can do this because he's divine, and we can't because we're not. But even a little later (Matt 18) Jesus commands his followers to forgive others. So if the authority to forgive is freely given, what is the point of a substitutionary punishment?
It's a simple thought, I concede, but is enough to cause a problem with the doctrine of penal-substitution.