By Robert Peston
Business Editor, BBC News
Investing in poker firms can be riskier than playing the game
When Partygaming was floated on the stock market last year, its prospectus admitted that its online gambling activities were deemed "illegal" by relevant US authorities.
Which was a pretty extraordinary thing for it to say.
Because most of its revenues come from US gamblers and it was seeking to persuade the stock market to value its business at a cool £5bn.
If most of its operations were illegal, why wasn't PartyGaming worth zilch?
Well, its prospectus also said that it directors took comfort from the "apparent unwillingness or inability of regulators generally to bring actions" against it.
Not any longer.
Congress has now passed legislation, which - if approved by President George W Bush - will make it impossible for Partygaming and a clutch of other British online betting businesses to collect revenues in the US.
Their share prices have collapsed - in Partygaming's case, by more than £2bn today.
But here's the bitter twist for gamblers who have lost money on shares of Partygaming and its rivals.
Congress has permitted online gaming by companies providing horserace betting, so-called fantasy games and lotteries.
And guess what? Most of them are US-owned.
It is British businesses that are being crushed by Congress.
Which suggests that in the casino of globalisation, Congress may be engaged in old-fashioned protectionism - or rigging the game in favour of US companies.