I have an interesting situation because my LCD monitor actually refreshes at 120hz regardless of my computer's refresh rate setting (apparently it was designed to reduce the "ghosting" effect some lower-end LCD's have).
I figure that having a 60hz refresh rate set on my computer is actually limiting the framerate. True, the game itself may actually be running at 75+ FPS, but if the monitor is only refreshing 60 times per second, then your framerate is effectively reduced to 60, right?
With that in mind, of course you would hardly be able to tell the difference between 100 FPS and 30 FPS on a monitor, because the monitor is displaying a solid 60-75hz anyway. I believe the main contributor to playback smoothness would be V-Sync, to make sure the action moves at a constant speed.
To make my point, all DVD's are recorded at 24 FPS, yet the action looks smooth enough because of motion blur. Yes, you would be able to tell a difference between 24 and 60 FPS, because action sequences (or anything involving fast movement) would look much cleaner at 60 FPS, due to less motion blur being needed to simulate the same effect. However, in my opinion FPS rates above 60 don't make as much of a difference when talking about fluid motion, but rather makes movement seem clearer.
Don't get me wrong though. Games in my opinion require at least 60 FPS to display smoothly. I don't think it's because of the actual framerate though, but rather the lack of motion blur.