The ruling revolves around the regulation of carbon dioxide, classifying it as a pollutant and, more specifically, a greenhouse gas. Considering that this is the first case involving global warming, the 5-4 ruling was a major blow to the current administration's opposition of mandatory controls on emissions.
Some of the quotes from the court are worth a read (click here for AutoblogGreen's post), but in the end, it's not the court's place to make regulations, only uphold them. As such, the EPA has to make changes to the existing law and/or uphold current regulations. However, don't expect anything to change until later this decade, if and when the Executive Branch has a shift in its policy towards global climate change.
[Source: Reuters via AutoblogGreen]