Income inequality is bad for rich people too - Glenn Greenwald - Salon.com: "Given that this rise in wealth has been accompanied by an increase in the power of those at the top, is there any hope for achieving a more just society? Bizarrely, the self interest of the upper crust argues in favor of it. Profoundly unequal societies are bad for everyone, including the rich."
Yves does a nice job summarizing some of this research, that I have covered in the past. See also the references (including a great video) here