Well, the ideal would be very strange though. Top 20% owns only twice as much as bottom 20%? That would mean that the upper class - doctors, lawyers would earn only twice more than garbage men. That probably wasn't true even in Soviet Union

EDIT: played with it a bit and produced this chart - Y axis has the base of the exponential (top 20% earns n-times more than bottom 20%). It seems that what US has is not exponential though - I can't get the bottom 40% to be as small without the top 20% becoming too big. I think that means that it's subexponential

http://i.imgur.com/gHxBXwa.png

EDIT 2: This is actually about wealth inequality rather than income inequality. So it makes sense for that to be much larger. Here is the paper it's based on

http://www.nber.org/papers/w18559.pdf