Iran is now selling 65% of its oil in euros, 20% in yen, and only 15% in dollars, and
even that 15% it plans to switch to "more creditworthy currencies" such as the UAE dirham.With the Bush administration threatening fullscale war against Iran almost every day, and taking any number of other warlike steps (sanctions, funding of terrorism, etc.), political reasons are enough to explain this pricing decision, but as an Iranian spokesman noted, there is also "the fluctuations of the dollar on the currency markets and the depreciation of its value since 2004."
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(Note: also see original article here)
By the way, the dollar has been tumbling for a long while already. Chek out this graph from the US Dollar Index USDX, a measurement of the strength of the Dollar against six other freely exchangeable currencies (for an explanation of the USDX check here):
(click on image to enlarge)