Pray that it doesnt happen, for it is does, the worlds economy will tank like never before. This issue alone is worth going to war with iran over.
Oh, horse-fudge. FUD hawking and nothing more.
The mother of all tanks? Sorry! I couldn't resist..
A lot depends on how swift or gradual it were. If it were swift, it would be very ugly for everyone involved, which is why it won't be. The Europeans don't want it to be swift any more than anyone else.
Now, if it were gradual, that's a whole nother matter. The most plausible scenario I think for a transition would be using both for a time, with a steady switch over to euros. In that case, the biggest *long term* effect would be that the United States would need to seriously reduce its deficit spending. In short, it would become a more 'ordinary' economy.
There would be other effects too, but that would be the most crucial. It's worth keeping in mind that Japan and Europe have highly developed economies without the support of the oil dollar benchmark.