It's because he has very little *earned* income, but has a lot of dividends and capital gains - taxed at 15%.
See Greg Mankiw's (who is one smart econ mo-fo) article on the subject:
http://gregmankiw.blogspot.com/2007/06/mr-buffetts-tax-bill.html
O.K., I'll buy that. AND, I don't have a problem with it. All that money he has invested that makes dividends and capital gains for him is working for America too. That is the capital businesses use to expand and hire people people that pay taxes, or banks lend to folks to buy homes that fuel the construction industry, or is put into annuities so insurance companies can insure a single moms life. His investments are profoundly good for America and anyone that doesn't see that is an economic retard. Those investments have to be encouraged, which is why it is taxed lower. Buffet nows it is good for America and other Americans. He must make the argument out of some sense of guilt. Hell if he feels he pays too little he can make a donation to the Treasury in my name.