The Supreme Court of the United States has, in a 5-4 decision (pdf), just ruled that the Environmental Protection Agency's decision not to regulate carbon dioxide as a pollutant under the clean air act was, "arbitrary, capricious, or otherwise not in accordance with law."
The decision does not require the EPA to begin regulating greenhouse gasses, but it does send the case back for further considerations, and it does tell the EPA that none of the justifications that it attempted to use to avoid regulating CO2 is a legitimate basis for refusal. The ruling clearly has massive implications in any number of areas, and I'm sure that more Sciencebloggers will be weighing in as the day goes on.