Previously on Paul.
Accordingly, the picture of God's anger is probably much more to help our understanding than a literal picture of God. It is an "anthropomorphism," a humanizing of God that helps us catch a glimpse of something beyond our comprehension. It is God's baby talk to us in terms we can understand. The Old Testament in particular uses these sorts of images, like God's nose getting red with anger or him being jealous or changing his mind. 
As with all such pictures, we can take the illustration too far. An example is the very widespread emphasis on something called "penal substitution" in some parts of the American church today. Certainly Christians believe that God is just, that God will one day punish the wicked and reward the righteous. Under the influence of Paul's letter to the Romans, Western Christianity in particular has come to emphasize that everyone is wicked and no one is righteous. God's justice thus dictates that he will one day punish everyone.
However, we have seen in Romans an even greater emphasis on the "righteousness of God" or, as James 2:13 puts it, that "mercy triumphs over judgment." God is both merciful and just, but the New Testament regularly sees the merciful aspect of his character as primary and the "judgmental" aspect of his character as secondary. We thus should reject the common picture of God we often hear today of a God who is a slave to his justice. The fundamental question here is this--did God have the authority simply to forgive us without Jesus dying on the cross or did someone have to pay for us to be forgiven?
It makes much more sense to see Christ's death more as a demonstration of God's justice--which is how Romans actually states it--than as a necessity of God's justice. In the Parable of the Prodigal Son in Luke 15, the father does not have to find someone to pay for the sin of the son. He has the authority simply to forgive him.  Any picture of God that is less than God in this story is a skewed one.
And so the modern view of penal substitution has taken the anthropomorphism of the Old Testament and the secondary emphasis of the New Testament and has blown the image of God's anger into a skewed picture indeed. God is now a slave to very rigid rules of justice. Someone has to pay--God cannot simply forgive someone. And it has to be the party that did the offence--no one can pay for us. So Jesus has to become human so that one of us is paying and yet indeed someone is paying the bill of justice. And, yes, every last dime of the bill must be paid.
Suffice it to say, this is a skewed picture of God that has little basis in the New Testament. Rather, the God of the Parable of the Unforgiving Servant has the authority simply to wipe the slate of the servant's debt clean on his own authority. In the New Testament, God sends Jesus to reveal God (John 1), to preach the good news to us (Acts 10:36), to free us from the slavery of death (Heb. 2:14), and, yes, to die for sins (Rom. 8:3). But nowhere is the image of Jesus dying for sins developed along the lines we have been mentioned.
This picture of God has implications--bad implications for how we relate to others, raise our children and so forth. It can foster an attitude that is unhealthy, even devilish. Justice is indeed the default. Wrongdoing should have consequences and even when a person is truly repentant for doing wrong, it can still be important to experience consequences. Something about the order of things, something about human nature truly benefits from such consequences.
But such consequences are beneficial. They actually help us. It is the difference between discipline and punishment. The legalistic sense of God's justice fosters an attitude of punishment. I know this feeling as a father, the anger of justice that can well up inside of you because of a child's open disobedience. It says, justice must be satisfied, and it pushes to immediate and swift punishment.
But Jesus' death on the cross was not some legalistic playing out of God's justice. It is a demonstration without which it would be very difficult to experience the depth of our alienation from God and the depth of God's love for us. It is not God making sure someone gets it so that his justice is satisfied. It is God reaching out to us in the most powerful way possible to show us the disorder of our world and the magnitude of setting things back in order.
Over the years as a parent I have learned about God as I have learned to be bigger than the affrontery of disobedience. Can I see in my child's disobedience his or her need rather than my offence at being disobeyed? In the public schools, in society at large, when we see defiance and wrongdoing, can we see with compassion a world that desperately needs God's help? I have come to see that the God who is out of control and must swiftly punish sin is not a big enough God. It is a Zeus.
The Christian God is not this god. The Christian God is a God who is too big not to be able to handle our disobedience. He is a God who looks on us in compassion when he sees how stupid we are to disobey, how we are hurting ourselves in our ignorance. He is big enough to take it. Indeed, he made the world in such a way that we would be able to choose whether to obey or disobey him. He is a God who disciplines to make us better, not who punishes because cannot help himself...
 If God knows everything at every point of his existence, then he cannot literally change his mind. Such pictures of God were very intelligible to the ancients, and God used them to speak to ancient Israel. However, from the standpoing of fully developed Christian understanding, they are still too much like a Zeus or a Marduk.
 And here let us add to our list of books to master Paul Joel D. Green and Mark D. Baker, Rediscovering the Scandal of the Cross (Downers Grove, IL: IVP Academic, 2000).