I really like this site. Being a newbie, I apologize if the following has already been addressed.
I looked and saw several movies I enjoy mentioned in discussions, but not the 1984 Dune. The stylized ships, dwellings and, various apparatus alone seem to point to Steampunk, but also the Victorian dress the females wear on the emperor's planet seem to scream Steampunk. Plus there seems to be literal steam coming from everywhere, at least in the 1984 movie. I say, "In the movie" because I have not read the book and I'm sure there are differences.
I also realize this takes place in the future and labeling Dune as Steampunk would cause conflict for some considering the timeline. However, I believe Dune can still be called Steampunk or maybe Neo-Steampunk.
Most people hear "old" music and automatically lump it in as "Classical" when in reality it may be Medieval, Renaissance, Baroque, Romantic, and a small sliver of time called "Neo-Classical" (sometimes called Modern) which is my favorite to listen to and was my favorite to play when I could. It sounds like classical but was written in the 19th and 20th centuries.
The back story in the narrative, at least in the movie, tells of the rise and fall of man, then the rise again. This could put them in a "Victorian-like" era and thus – Steampunk.
I love the movie and no negative comments could sway me otherwise, but I would be interested in thoughts about Dune falling under the category of Steampunk. It would be nice if those that post comments would have actually seen the movie, but hey, comment away.