Sometimes US foreign policy is conducted using hidden messages (in order to make sure US citizens have no idea what's going on) but I believe Iran knows what the message is: the United States will rain down death and destruction on Iran (just as we did in Iraq) if Iran moves ahead aggressively with its effort to open an oil exchange where contracts will be paid for in euros rather than dollars.
On the other hand, Iran is telling the US that if you make war on us, we might choose to sell our oil only for euros; dollars will no longer be accepted.
So what we have here is a game of chicken.
If the US and/or Israel goes ahead and bombs Iran under the pretext of stopping their atomic energy program, Iran might refuse to sell its oil for dollars. This probably would precipitate a world wide economic recession with the US economy being among the primary losers. That might be an understatement. Of course we have our fleet now in the gulf to place an embargo on Iranian oil if they were to do that, but how do we know that Venezuela would not follow Iran's lead?
Of course, Iran doesn't want to be bombed back to the Stone Age as we have now done in Iraq, and its leaders are not much interested in regime change, an activity in which the US has some expertise and a history of success.
This would not be the first time that the US government set out to overthrow the Iranian government. The US was successful in overthrowing Iran's legitimate and democratically elected prime minister, Dr. Mohammad Mossadegh in 1953, replacing his government with a repressive puppet regime that was favorable to British and US interests. The reason we took it upon ourselves to destroy their emerging democracy in 1953 was that Mossadegh had decided to nationalize the oil industry rather than continue to allow what was then Anglo-Iranian Oil Company (today known as British Petroleum) to rape the country of its oil resources. In any event, the dictator Mohammad Reza Shah Pahlavi (The Shah of Iran), who we put into power to protect our source of cheap oil, was eventually overthrown by Islamic radicals, who were able to gain public support due to the Shah's excessive repression. It's their successors who we are dealing with today. A great many Iranian citizens probably remember or know what we did to them and their country in 1953 when we killed their chance to live in a democracy.
I suppose it's sort of a stalemate at this point. Until the US or Iran backs off or escalates, we are held in suspense. If it comes to war, I don't see how either side can win. If Bush really believes the US is as powerful and invulnerability as his rhetoric implies and he acts on his illusions, it will be very interesting to watch.
I will leave it to other experts to explain why trading oil for Euros would be so devastating to the US economy. The truth is we have no moral right to the exclusive monopoly on how oil transactions are executed, just as we have no legal or moral claim on the oil that lies beneath the sands of the Middle East.
Our vulnerability is due to our own greed, negligence, and malfeasance, which is in turn reflected in the people we elect to office and many of those who run our corporations.