Actually, 82abn, only one repented, while the other berated Jesus as much as the mockers on the ground did. A clear illustration of the choice we all have: accept or reject the Son of God.

As for the parable of the "Prodigal Son" DaneCook is referring to, I have seen it interpreted two ways:
One interpretation holds that we are all children of God, and the son who leaves and squanders his inheritance is a lost person. When he finds that the pleasures of the world have nothing to offer him, he comes to the realization that he needs Jesus (the Father) in his life and repents of his sins and is thus saved.

The second interpretation, and the one I hold to, is that the son who departs is already a Christian since he is listed as a "son". His sojourn into sinful activity ended in what all Christians know to be true: If you're not busy going about the Father's work and are about the things of the world, you're going to be miserable. The father never stopped loving that son and apparently kept an eye out for him, because he saw his son returning and ran to meet him. That's how it is when we sin and repent: God is faithful, and eager, to forgive because He wants that close parent-child relationship with all of us.
As for the disgruntled son, do you know Christians who seem to delight when one of their own falls from grace, and is angry with God for forgiving them? I've certainly seen that happen in my lifetime.
In conclusion, God's first point to us who serve faithfully is that we should rejoice for those who have lost their way but found it again.
The second point is that we should never take the self-righteous attitude as that of the second son, as the Father is well aware of who does what.
Jesus made it a point in the Bible: we should be concerned with our service to Him, and not at looking at what someone else is doing/has done. He rebuked one of His disciples, who wanted to know what other disciples were going to be doing. Jesus essentially said , "That's none of your business, you focus on what I have told you to do." On another occasion when addressing the sinful activity of a person, He tells us to clean up our own act before we try to clean up someone else's. (Loosely translated)"How can you see to get a little speck of sawdust out of your friend's eye when you've got a 2x4 stuck in your eye!?!?"

I guess that is as clear as mud? Anyway, I think the bottom line is that we need to focus on what we're doing. After all, He doesn't expect results from us....He only expects obedience.