I disagree with the idea that metaphysics doesn't belong in psychology. All too often, people tend to draw metaphysical conclusions from psychological research or ideas. These ideas can profoundly influence the direction of that person's further psychological inquiry, and so I think that a developed system of metaphysics will help direct and guide the scientific enterprise that modern psychology is. Also, the nature of psychology is such that often scientists go to great efforts to create operational definitions for concepts that mean so much more than the mere observable components that are tested, but it's not science if you can't take objective measures. Philosophy and psychology are inseperable.
I think it deserves at least one thread, at least as it pertains to psychology, otherwise I think Descartes would not be pleased...