Because the human eye has an effective “refresh rate” of around 30 frames per second, there is a certain point at which even major increases or decreases in frames per second will have no visible effect. I’d place that number, off the top of my head, around 100fps — at that point your eye isn’t going to “catch” the display in mid-update, as the refreshes will happen faster than our visuo-temporal granularity — so movement appears completely continuous and no interpolation is done by the eye, which interpolation, paradoxically, is what makes 24p such a convincing frame rate for film.
But all those big words didn’t stop Sony from showing GT5 at 240fps, though! Using four PS3s working in parallel (like in this nearly 4K resolution demo), their “Field Emissions Display” or FED took frames from each PS3 in turn, displaying at a total of 4 x 60 = 240fps. The people there were reportedly awed by the smoothness of it, but honestly play a game at 30fps, then play it at 60 and see the difference, then go up to 90 — not much better. The higher you go, the more your returns diminish. I’m glad they have a display that is capable of it, it means less flicker and Vsync that doesn’t limit your framerate, but somehow I’m guessing this won’t be a household technology for a couple years.