Exactly. While it's hard to get the exact figures (as CEOs graduated over a long time period), we can make at least some approximations.

I just run quick numbers for the year 2007:

  57,845 Ivy League undergraduate enrollment [1]
  15,386,000 Total US undergraduate enrollment [2]

Which makes Ivy League population just 3.76 promille of the total US undergraduates.

Thus Ivy League gives you around 27x higher chance to be Fortune 500 CEO compared to what you would get if the selection was random.

[1] http://en.wikipedia.org/wiki/Ivy_league

[2] http://nces.ed.gov/programs/coe/2007/section1/table.asp?tabl...