For what it's worth, Larry Rosen argues that use of a unmodified GPL'd work, as specified in its API, doesn't necessarily create a derived work. In my opinion,

For what it's worth, Larry Rosen argues that use of a unmodified GPL'd work, as specified in its API, doesn't necessarily create a derived work. In my opinion, this question won't ever be put to rest since it's about specific cases, law and human interpretation. It matters quite a bit about what the GPL'd work is, what the application is that uses it, how they are related, and the public intent of both parties. While some lines may form in the future, the boundary of safe use itself will always be a bit blurry.