Today, the EPA issued a new proposed rule to fix the flaws in its Clean Air Interstate Rule (CAIR), which the D.C. Circuit vacated, but allowed to remain temporarily in effect, in North Carolina v. EPA, 531 F.3d (D.C. Cir. 2008). The proposed rule can be read here. According to EPA's regulatory impact analysis for the proposed rule (here), the social benefits would exceed the social costs by up to a factor of 10.
If adopted as promulgated, the rule is sure to be challenged in court. The question will be whether the agency has done enough to meet the D.C. Circuit's prior objections to the CAIR. I may have more to say about this issue after I've had a chance to more carefully read the entire proposed rule, which runs to 1,361 double-spaced pages.