No. 12 -- John Arnold, Centaurus Advisors

NET WORTH: $3.3 billion

Arnold, who made his early fortune in Enron trading bonuses and turned those into one of the most successful hedge funds in the U.S., is one of five hedge fund managers on the list.

Arnold is not only among the richest Americans, but among the youngest billionaires on the annual Forbes ranking at the age of 36.

Sometimes referred to as the king of natural gas, Arnold's energy trading is arguably most famous for reportedly being on the other side of the trade that sank Amaranth Advisors. Arnold also bought 10% of National Coal in 2008.

Arnold's summary of his big-hearted plans reads as could only be written by a trader, particularly points 2 and 3: "We focus on areas where (1) philanthropic investments can lead to solutions that are self-sustaining in the long-term, (2) we can leverage a relatively small investment to create a large impact on total societal benefit and (3) the market does not presently yield optimal results, due to inefficiencies, lack of adequate information or other reasons."

Among notable absences on the Buffet/Gates list at a much higher income level than Arnold are the ninth-richest Americans as ranked by Forbes -- the Brothers Koch, Charles and David, who run one of the largest private companies in the U.S., Koch Industries -- along with the two executives who tied for No. 11. Go and Google the Buffett/Gates list you'll see that neither Sergey Brin nor Larry Page, Google's ( GOOG) founders, with fortunes over $15 billion each, are among the initial billionaire pledges lined up by Buffett and Gates.

Of course, some wealthy don't wish to turn charity into an occasion for personal publicity, while others might prefer to manage their own philanthropic affairs and on their own schedules, apart from a nudge provided by Gates and Buffett. That's the case with many notable U.S. billionaires not taking the Buffett/Gates pledge.

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