From that link:
"
Because crude oil contracts are currently traded in dollars, and the prices of OPEC crudes are determined by using complex formulas derived from marker crudes, such as Brent and WTI, there is not much the Organization can do unilaterally until, and unless, there is a switch of denomination in these markets. OPEC has no control over the quotations of these marker crudes, whereas, in the past the Organization did set the official selling prices. That has all changed with the introduction of market-related prices which saw the system change from a seller’s to a buyer’s market, or at least where market forces now dictate prices. Moreover, the entire infrastructure of the oil market has been based around the dollar, and that will be hard to displace. However, as previously mentioned, a lot depends on Britain and Norway in determining what their level of EU integration will be, and whether their marker crude, Brent, could be traded in euros."
Since the original post and my answer, we have had the Dubai Ports debacle, and word today is that Dubai is going to divest itself of Dollars (if you can't invest them in the US, why have them?). This could lead to a trend among Gulf States.
But for a long time to come, contracts on the NYMEX and the London IPE will dwarf those of any Iranian bourse, meaning complicated FOREX calculations for anyone wanting to trade on both (hedging, arbitrage, futures, etc).
In addition, the Chinese Yuan is pegged to the dollar and India is a very large trading partner of the US. Niether will want to see a collapse of the dollar and would be hesitant to switch.
Don't forget that stock-exchanges of any kind have to have laws, rules, and a judicial system to back them up (who are you going to go to in Tehran if someone renegs on a contract? What kind of justice are you going to get?).