George Steinbrenner today became the fourth known U.S. billionaire to die during 2010, the first year in decades with no hefty federal estate tax. The timing of his demise could save his family hundreds of millions of dollars—-unless the estate tax’s one-year suspension is repealed retroactively to Jan. 1.

Forbes figured Steinbrenner, who turned 80 on July 4, was worth $1.15 billion. Almost all of that was the estimated net value of his share of the New York Yankees--the baseball team he bought from CBS in 1973 for a mere $10 million--the new highly leveraged Yankee Stadium and a regional cable network.

Undoubtedly, tax and legal advisers have been working for years to minimize any estate tax bite from the passing of Steinbrenner, who long has battled an array of health woes. Likely strategies would have been the gradual transfer of economic interests to later generations-- –the team is now run by two sons--and the use of partnerships and other legal entities to take advantage of laws allowing valuation discounts for fractional ownership.

Steinbrenner is survived by his wife, Joan, and four children. Under federal law, probate transfers among spouses are tax-free. But the absence of the estate tax could set up an intriguing scenario in which Steinbrenner’s spouse could disclaim (or turn down) a bequest, allowing assets to move to the next generation.

The absence of a current federal estate tax dates back to the tax cuts launched by President George W. Bush in 2001. Congress approved laws cutting the estate tax in stages and whacking it totally for 2010. For 2009, the rate was 45%. A 55% estate levy with a mere $1 million exemption is schedule to return for deaths on or after Jan 1, 2011.

The first U.S. billionaire to pass away this year was Houston oilman Dan L. Duncan. He died at age 77 in March leaving behind a fortune we estimated at $9.8 billion—more than eight times our view of Steinbrenner’s net worth. His heirs likely saved more than $4 billion in estate taxes.

Last month, California real estate mogul Walter Shorenstein died at age 95. We pegged the net worth of Shorenstein and his family at $1.1 billion.

In February Mary Janet Morse Cargill, from the Minnesota family that founded Cargill Inc. died at age 85. Forbes figured her net worth at $1.6 billion.

Duncan’s death was cited in a letter last month by three liberal U.S. Senate members calling estate tax reinstatement retroactive to Jan. 1. “At a time when we have a record-breaking $13 trillion national debt and an unsustainable federal deficit, people who inherit multimillion- and billion-dollar estates must pay their fair share in estate taxes," said the letter, signed by Sens. Bernard Sanders, I-Vt., Tom Harkin, D-Iowa, and Sheldon Whitehouse, D-R.I.

They have introduced a bill that would put a 65% tax on big estates. Legal scholars debate whether a retroactive imposition of a tax is constitutional, although some past court rulings have upheld such legislative actions if they don’t go back further than the current tax year.

Steinbrenner’s legal residence was in Florida, which has no state income tax and a light taxing structure. That might allow him to duck much of the relatively heavy 16% state estate rate in New York, where the Yankees are located.