As promised, a post about everybody's favorite topic: semi-Pelagianism! I know you're excited.
Before I talk about semi-Pelagianism, it's worthwhile to talk about regular Pelagianism. Pelagianism was a heresy in the church that basically claimed that human beings could (and indeed, had to) earn their salvation by doing good works. The argument went like this: God is just and fair. Therefore, God would not command us to do things that we were incapable of doing. Therefore, it must be possible to fulfill all of God's commandments. Therefore, we must fulfill all of God's commandments. (Therefore, etc, QED.) This heresy was put down, but its descendant, semi-Pelagianism, challenged the church as well. This form of Pelagianism is still present in the modern day. Semi-Pelagianism is willing to deny almost everything in Pelagianism, but holds on to one little point, and by that point falls into the same problem. In short, semi-Pelagians argue that God does everything necessary for our salvation except (and this is the problem) for taking the step to make it effective. In other words, God does everything for us, but we have to accept it through faith. It's described nowadays with images like, "God has done everything for you, he's opened the door, now you just have to walk through" or "God has done everything, it's all there, the water is hooked up to the faucet, you just have to turn the faucet on." The problem is, any formulation of salvation based on this model (God has done everything except...) boils down to the old heresy of Pelagianism, or what Lutherans call "works righteousness".
Why is this a problem? There are a lot of arguments that can be brought against Pelagianism, but I like this one: if there is any human action necessary for salvation, then God could come to a person and say, "I want to save you," and the person could respond, "No. I don't or can't believe or do what you want me to." In short, it makes God's power and will subject to human power and will; and while we don't have to accept that God is omnipotent (see my earlier post on the Greek view of God), we do affirm that God's power is incomparably greater than our own, so it cannot be subject to anything created.
In short, the opposition of faith and works is flawed in semi-Pelagianism because faith becomes a work. As Luther says (and Paul before him, of course), human beings are justified by faith, not by works—but that necessitates that faith must not be a work. Faith is not the result of human will; it is given by God. Now, if we ask why God gives faith, we get into another debate—whether faith is given to those whom God chooses to save, and not to others (predestination), or whether salvation is universal, and faith is given to some for the assurance of salvation (universal salvation). I'll leave that one up to you.