Or better yet, what effect ought precedent have? The recent ruling in the Eastern District of CA has many in the environmental community cautiously optimistic about improving fuel efficiency in the US. The ruling was a denial of summary judgment sought by the Ass'n of Int'l Auto Mfrs—an industry group that, according to its website, promotes competition, advances technology and invests in America (and who could be against all that?). What AIAM argued was that CA, in seeking to adopt auto emissions standards aimed at reducing greenhouse gas emissions, was preempted from doing so on several grounds. Each was rejected and, indeed, CA and its intervenor friends were granted their own summary judgment to the effect that, in the event EPA approves the "waiver of preemption" which CA has sought under the Clean Air Act, CA will be empowered to impose emissions standards on new vehicles. This may be in some tension with NHTSA's authority to balance energy efficiency needs against auto travel under the Energy Policy and Conservation Act. (As Mike pointed out in his Findlaw column today, there is a very strong correlation between emissions and fuel efficiency, although as some people seem weirdly diligent in arguing, emission standards and fuel efficiency standards are two separate things.)
What is most interesting about this particular D.Ct. decision, though (the fuel economy stuff is likely to be worked over in some substantial way by new legislation and/or a new administration), is the way in which the judge seems to have changed his mind about the preemption issues in light of Massachusetts v. EPA, 127 S.Ct. 1438 (2007). Specifically, Judge Ishii went from believing that the regulation of greenhouse gases as Clean Air Act "air pollutants" was too far a stretch under that statute to finding that this stretch was natural, logical and in keeping with Congress's intent. He did so in large part because of how SCOTUS interpreted the Act in Mass. v. EPA, which, of course, was directly contrary to EPA's view regarding greenhouse gases and their fit within the Act's structure.
My own hunch is that courts are losing interest in agencies' interpretations of their statutes whenever judicial precedent interpreting that statute exists. Moreover, this is making cases like Nat'l Cable & Tel. Assn v. Brand X Internet Servs. 545 U.S. 967 (2005)—cases that allocate interpretive authority as between precedent and agency judgment—much more important today than in the recent past. One response to all this is that judges back into opinions like Ishii's and that complicated allocations of authority have little to do with it. I have my doubts about that response as a substantive matter (the empirical support for such "attitudinal" models is weak), but as a pedagogical matter it is pure tripe. We should shorten law school down to a few days if that's what we ought to teach about the power of legal reasoning.