Billionaires only for Forbes' list of US super-rich

It's no longer enough to be very, very rich to make the Forbes magazine list of the 400 richest Americans. This year, for the first time, only a billion dollars will do.

The list, based on the magazine's estimations of personal wealth, showed the net worth of the richest Americans increased in 2006 from $1,130bn (£590bn) to $1,250bn.

The top two names on the list remained unchanged. Number one was Bill Gates, the founder of Microsoft, who was said to be worth $53bn. Next came his friend and bridge partner, the Omaha-based investor Warren Buffett, with $46bn.

The first big surprise was the man in the number three slot, Sheldon Adelson, who is chief executive of the Las Vegas Sands Corporation, which owns the Venetian casino in Las Vegas and a clutch of other gambling-related properties around the world. He is worth $20.5bn, up from $9bn last year, jumping from number 15. Shares in the Sands Corp have doubled since the company went public at the end of 2004. Forbes estimates that Mr Adelson has personally earned almost $1m an hour since then.

Most of the billionaires on the list are over 50, and only seven are under 40. Most prominent among the younger generation are Sergey Brin and Larry Page, the co-founders of Google, who are both 33. Their net worth was put at $14.1bn and $14bn, putting them in 12th and 13th place on the list. Google's chief executive, Eric Schmidt, is ranked at number 45. He is worth $5.2bn.

Much of the bottom half of the top 10 was take up with the heirs to Sam Walton, the founder of the Wal-Mart discount supermarket chain. Also near the top of the list were Paul Allen, Mr Gates' partner at the inception of Microsoft, Michael Dell, of Dell computers, and Larry Ellison, of Oracle.

Becoming super rich often involves a certain ruthlessness in business, and yet the top two names on the list are becoming increasingly well known for their philanthropic work - with each other. Earlier this year, Mr Buffett pledged 85 per cent of his fortune to the Bill and Melinda Gates Foundation and other charities.

One notable absence from this year's list was Martha Stewart, the lifestyle guru whose stint behind bars in a share trading scandal had a rocky effect on her business, Martha Stewart Living Omnimedia. Ms Stewart lost $395m over the past year.

Some of the names lower on the list were surprising in strange and eccentric ways. Number 242, for instance, is the Colorado businessman James Leprino, estimated to be worth $1.5bn. His company, Leprino Foods, is the world's largest producer of mozzarella cheese - a line of work worth $200m more than last year.

Most of the names on the list, though, were of coastal city dwellers, not heartland entrepreneurs. More than one in five on the list came from California, and about one in seven from New York.

The billionaires' row that makes up the Forbes list is a symptom of the gap between rich and poor in the United States. According to the US Department of Labour, the median income of America's 105.9 million full-time workers is just $34,268 per year.

A billion reasons to be cheerful

1 Bill Gates is still top of the pile with $53bn

2 Warren Buffett's investments have given him $46bn

3 Vegas property magnate Sheldon Adelson is worth $20bn

12 &13 Google founders and presidents Larry Page and Sergey Brin are worth $14.1bn and $14bn

16 Abigail Johnson, of Fidelity Investments, is the wealthiest working woman worth $7.4bn