No, the display always runs 120Hz refresh rate. But you can tell by watching (specifically slow pans) that you're getting 5:5 pulldown vs 3:2:3:2 you'd get if the video processor just doubled the incoming frames. I believe it's part of Samsung's AMP, it can detect film vs video content (since it applies AMP differently to each based on your settings). So I've got mine setup to run AMP on video rather strongly (which clears up scrolling) but basically not at all on film, but I still get 24fps-appropriately smooth pans.
Mine goes black fo 500 millisecs or so and then says 1920x1080 24p ina box in the upper corner. Is it actually doing 5:5 pulldown at 120Hz?
Sounds like your source is switching output frame rate. I've done that with my SageTV extender into my Samsung, and you'd see it have to resync to 480p or 720p, or 1080p60/1080p24, but since my Samsung is just in the living room for casual viewing, it's more annoying to have it switch resolutions on me than it is to just let my HD200 do all the video processing, even if the Samsung is better, so I just output 1080p60 for everything.