Warren Buffett does not have to worry: his income tax rate will be rising about 3.8% on January 1, 2013. He does not even need a new “Buffett Rule” tax: it is a part of the health care reform bill of 2010 (“ObamaCare”) that “unearned income” (meaning interest, dividends and capital gains like those enjoyed by Mr. Buffett) will pay an additional surtax of 3.8% on amounts over $200,000 for individual taxpayers. 
This means Mr. Buffett’s average income tax rate, which he says was 17.7% in 2010, will be approximately 21.5% under the 2013 tax rates. The new rate is already enacted as law. This fact is certainly relevant for any discussions of a “Buffett Rule” or higher tax rates on the affluent, yet it seems to be almost completely ignored in the media.
For more on Mr. Buffett’s misleading opinion piece, see https://econscius.wordpress.com/2011/08/19/trust-a-cpa-warren-buffet-is-wrong-about-tax-rates/ and https://econscius.wordpress.com/2011/08/29/is-warren-buffetts-924725-employee-super-rich/
Pictures from Wikipedia Commons. Disclosure: the author owned common stock in Berkshire Hathaway as of this writing.