http://www.irs.gov/pub/irs-tege/eotopick03.pdf

"IRC 501(c)(6) provides for exemption of business leagues, chambers of commerce, real estate boards, boards of trade, and professional football leagues (whether or not administering a pension fund for football players), which are not organized for profit and no part of the net earnings of which inures to the benefit of any private shareholder or individual."

Of specific interest is the "Statutory History" section on page four - a history of private organizations asking for exemptions.