I always heard that 50mm is the same as looking with your eyes, aka, moving the eye in and out of the VF makes for the same mag ratio, but it isn't, it's much closer to 75-85mm, I've never heard anyone explain why this is. Also read somewhere that the ACTUAL focal is closer to 43mm, which makes no sense to me.
As Lawliet stated, 'normal' is when the focal length is approximately equal to the diagonal of the film/sensor format. 50mm was 'close enough' to the 43mm diagonal of film/FF, and was easy to produce (although I don't think 43mm would have been siginificantly more expensive...). Calling it 'normal' wasn't just made up, however - when the lens focal length equals the image sensor diagonal, the final image tends to approximate the field of normal human attention.
Note that I stated final image. When comparing it to what you see in the viewfinder, there's another factor - viewfinder magnification. For example, the 5DII has a viewfinder magnification of 0.71x - so in terms of magnification, looking through a 70mm lens on a 5DII would give equivalent magnification to your eye,. explaining why, "...moving the eye in and out of the VF makes for the same mag ratio, but it isn't, it's much closer to 75-85mm
A FOV of the eye is 180 degrees, no? At least I can see 180, so it can't be that a fov of a 50 is equal to the human eye, it must be the mag ratio. And doesn't this have something to do with how the VF itself is constructed? how great coverage etc?
A little more - the total human visual field (both eyes) is about 200Â°, with about 120Â° of overlap (binocular vision). But even though you can see a 200Â° visual field, I'm sure you notice that objects away from the center are not sharp. In reality, you don't pay attention to the whole visual field - you really only use the very center of each eye's visual field, and when you look at a scene, your eyes rapidly move around the scene (called saccades), and your brain builds a cohesive picture from the individual, small parts of the scene. Probably the idea of a 'normal' lens comes from the fact that the FoV of a 45-50mm lens on a 35mm/FF camera approximates the area of the total human visual field that is usually scanned to build up that mental picture. If there's something of interest at the periphery, you'll turn your head to look at that area and build another mental scene.
So, jeremymerriam hit the nail on the head - a camera functions in a complately different manner than the human visual system. That fact is part of the challenge of photography - it's pretty common to see an amazing vista with your eyes, but when you snap a wide angle shot of that scene, it looks flat and boring, capturing nothing of the feel of the real scene. That's why composition is so important!