That company, ASX listed Tabcorp, has flagged a possible entry into the world's biggest economy, but it's unlikely to happen any time soon. Morgan Stanley analysts last week said they "do not believe an entry into the US market is imminent" but it "would not rule out the US market as a long-term growth opportunity" for the company.

That leaves Caledonia - originally a private vehicle of the eminent Darling family dynasty - as one of the biggest immediate winners in Australia from the ruling.

The under-the-radar fund is the single biggest shareholder in The Stars Group, a $7 billion-valued, Toronto and Nasdaq listed business described by Macquarie analysts last week as "the premiere [sic] online gaming operator with leading exposure to perhaps the biggest secular development this cycle, US sports betting".

The Stars Group, which dominates the online poker market, has been branching out. In April, it struck a $US4.7 billion deal to acquire UK based Sky Betting & Gaming from private equity group CVC and Rupert Murdoch-controlled pay TV business Sky. That deal gave it a significant presence in the UK sports betting market.

It also has a significant footprint inside Australia - it recently spent $315 million to acquire British bookmaker William Hill's local operations, and to lift its stake in CrownBet from 62 per cent to 80 per cent.

Caledonia acquired 4 million shares in Stars back in June 2015, records in S&P Capital IQ show.

It has steadily increased its position since then.  By the end of April it held more than 29 million shares, or 19.5 per cent of the company, worth $1.3 billion at press time.

Since the Supreme Court ruling, The Stars Group's market value has increased by about $1 billion, putting Caledonia's gains, on paper, at nearly $200 million.

Caledonia is known for its highly concentrated investments - it only invests in a few companies. It famously made a killing on an investment in Zillow, a US online real estate business once backed by James Packer.  So big and successful was Caledonia's Zillow investment, in 2015 it decided to split off its stake into a separate fund.

The company's New York co-chief investment officer, Mike Messara, is believed to be behind the Stars Group investment. He didn't respond to an emailed request for comment about the investment.  Will Vicars, its Sydney-based co-CIO, also didn't respond by deadline.

The Supreme Court ruling ends a bizarre situation in the US where sports betting existed as a multibillion-dollar shadow industry (that has even spawned a multibillion-dollar quasi-gambling offshot - Daily Fantasy Sports).

Technically illegal, yet still widespread, betting on sport has grown increasingly mainstream, to the point where betting odds and handicaps are openly discussed by broadcasters during games.

According to at least one estimate, $US150 billion was wagered illegally by Americans on sport last year.

Individual American states are now free to decide whether they want to legalise sports betting. New Jersey, which initiated the Supreme Court action, could move to do so in a matter of weeks.

The ruling has understandably upset those concerned about the possible impact of gambling on the integrity of games, and those concerned the impact of gambling advertising on broader society.

But it represents a win for New Jersey governor Chris Christie, who staked a lot politically on the ruling, as well as the leagues (who want to earn commissions on bets), and even media companies such as Disney who could benefit from increased advertising spending.

And, much closer to home, Caledonia, and its ultra wealthy investors.