One thing that particularly intrigued me is that proponents of the GPL says that whenever you add code, you automatically grant permission for that code to be copied (a true enough statement). But what happens if you add some of your code (and thus grant permissions), whilst someone else copies some of your proprietary code?
This seems to be the nub of SCO's arguments; we added X and owned X and Y, but someone else added Y without our permission. Where does the GPL stand on that? If X were contributed to a different project, then it would be direct violation of SCO's copyright (by whoever contributed Y). However, if they're on the same project, does that automatically give Y to the GPL? I don't think it does, or should. If it does, it opens up a whole pandora's box -- if (say) IBM contributed to the Linux kernel with a patch, and then someone else stole some IBM code and put it in the kernel, would IBM have to cede to the GPL?