Hmm, this is a good question. After spending some time thinking about this, I think the problem I have in trying to separate "history of philosophy" from "philosophy" is that such an enterprise almost appears antithetical to the goal(s) of philosophy.* Philosophy seems meant not to be useful or practical, but intended to ask the right sorts of questions, think about things one abstraction deeper/more meta, and question things others don't question. As such, studying the history of philosophy is philosophy--and vice versa--insofar as the goal of philosophy is not to positively answer the right questions but to think philosophically and ask those questions in the first place. So, learning why Aristotle's ideas on physics are wrong is simply not the sort of thing with which philosophy would concern itself--for better or for worse.

*Thinking about it some more, I just realized that I may be conceiving of the goal(s) of philosophy as something different than what most of the posters here do. I get the sense that lukeprog (and others here) wants philosophy to provide answers to the deep questions, or at least attempt to do so. The problem is philosophy is not about that; maybe it should be, but then I'd argue that such a field is precisely what science is, with philosophy as almost a check/balance (making sure that the right questions are still being asked, assumptions questioned, etc.).