Arthur Laffer has a piece in the Wall Street Journal on the "Buffett Tax":
Mr. Buffett's donation to the Gates Foundation goes to the heart of my critique of his public call for higher tax rates on the rich. Just look at the second contractual condition for his ongoing pledge to the Gates Foundation: "The foundation must continue to satisfy the legal requirements qualifying Warren's gift as charitable, exempt from gift or other taxes."
In other words, if his gift weren't tax sheltered he wouldn't give it. So much for "shared sacrifice."