I've heard it suggested that OPEC's actions to limit oil supply violate WTO rules. See, for example, this recent press release and report from Senator Frank Lautenberg. After running through the legal arguments, the report concludes:
OPEC’s practices are in violation of WTO rules prohibiting quantitative restrictions on exports [under GATT Article XI]. Although there are exceptions that OPEC could attempt to cite, the applicability of these exceptions is tenuous. A WTO case against the six WTO members of OPEC could have immediate, large and lasting benefits to the US consumer and economy by driving down oil and gas prices.
In the press release, the Senator "called on President Bush to file an action in the World Trade Organization against the cartel’s plan to limit crude oil production, which violates Article XI of the General Agreement on Tariffs and Trade (GATT)."
So, is OPEC taking this as a serious threat? Well, it's hard to say for sure, but today as I was reading the Economist I happened to flip through their job advertisements. There I saw an ad from OPEC seeking a "Senior Research Analyst for WTO Matters." I checked the OPEC web site, and here's the position: http://www.opec.org/home/vacancies/Other_vacancies/WTOSenResAnalyst.htm . It's hard to say if the two are related, but perhaps somebody at OPEC took notice of the report.