Where Did 1280x1024 Come From?

Alan Shutko asks: "I was playing with different resolutions recently, and got confused. 640x480, 800x600, 1024x768, 1152x864, 1400x1050, 1600x1200, they all have a 4x3 aspect ratio. But 1280x1024 has a 5x4 aspect ratio. What's up with this? Somewhere in the annals of computing history, someone must have come up with 1280x1024. Why did they choose such an odd aspect ratio?"