This reminds me of the article a few years ago that indicated that Warren Buffett had a lower effective tax rate than his secretary, even though he made heaps more. http://www.timesonline.co.uk/tol/money/tax/article1996735.ec...
At least he doesn't pay FICA taxes on that. On the other hand, the capital he invests does allow companies to hire more people, who do pay taxes.
The utility-maximizing way to tax Mr. Buffett is at 0%. He'll invest it effectively, it will grow the economy, more jobs will be created, and we'll end up with more wealth to squander through silly redistributive schemes.