Had no idea it was based on a short story. Just finished reading it online... actually not a bad story at all. Granted, it's very dated now but I liked it.
Not sure it bothers me that folks don't know about the short story. In this case, I kinda feel like the movie was actually better than the short story that inspired it. The short story reminds me sort of the structure of one of the old episodes of the Twilight Zone. Still very cool though. And I'm still trying to figure out how to pronounce "Gnut"... nut, gah-nut, gah-noot? I wish stories would come with pronunciation guides sometimes...
But the movie and the short story really don't have a lot of similarities beyond the basic concepts... big robot, Klaatu, encasing the robot in a block of clear material, etc.