This could be very interesting:

The Supreme Court agreed Monday to consider whether the Bush administration must regulate carbon dioxide to combat global warming, setting up what could be one of the court's most important decisions on the environment.

The decision means the court will address whether the administration's decision to rely on voluntary measures to combat climate change are legal under federal clean air laws.

...

The administration maintains that carbon dioxide -- unlike other chemicals that must be controlled to assure healthy air -- is not a pollutant under the federal clean air law, and that even if it were the EPA has discretion over whether to regulate it.

A federal appeals court sided with the administration in a sharply divided ruling.

One judge said the EPA's refusal to regulate carbon dioxide was contrary to the clean air law; another said that even if the Clean Air Act gave the EPA authority over the heat-trapping chemical, the agency could choose not to use that authority; a third judge ruled against the suit because, he said, the plaintiffs had no standing because they hadn't proven harm.

The States core argument appears to be that carbon dioxide, as a deleterious chemical present in the atmosphere, falls under the Clean Air Act, and the EPA has a responsibility to regulate it like any pollutant.

Climate politics aside, the federal government does have a point when they say that CO2 isn't a pollutant. Pollutants, at least as I understand the term, are compounds which are directly biologically toxic. CO2 doesn't fit that criterion. I see how you could argue that CO2 is indirectly damaging, but it sounds like stuffing a square peg through a round hole.

As much as some states would like to force EPA regulation, I doubt the remedy is going to end up being judicial. My suspicion is that it is going to require a change of heart in Congress or in the Administration.

In any event, this is definitely a case to watch.