I think you are completely missing the point the top 20% _are_ dramatically pulling away; in the last ten years _all_ the gains in income have gone to the top 20%!
No, look at the data more closely. The top 20% numbers are skewed by the fact that the top 1% is included. The real story is the top ~1% pulling away from everyone else, NOT huge gains in the top 20% as a whole (another way to say that is that someone in, say, the 81st percentile of the income distribution has not seen large income gains)
edited to add link to CBO inequality report:
http://www.cbo.gov/publication/42729
(shows that 1979-2007 top 1% income grew by 275%, the next 19% grew by 65%, next 60% grew by 40%).
As I said before, there is certainly a structural component that means that education and skills can command even more of a premium than they always have, but that's not the majority of the story behind this inequality growth.
