Until this one. In December 2005, after finalizing the details of its emissions rules, California submitted its waiver request to the EPA. It didn't hear back from the agency. After two years of delay, in November 2007 California sued the agency, and their suit was joined by 13 other states. The following month the EPA officially denied California's application, with the agency's administrator, Stephen Johnson, saying that he wanted to avoid a "confusing patchwork of state rules."
In May 2008, following a Congressional hearing on political interference in the EPA, Congressional Democrats released a report [PDF] saying that the waiver had unanimous support among EPA career staff and that Mr. Johnson himself seemed to support the idea, but that he changed his position after hearing from the White House. Johnson maintains that the decision was his alone.