Here is the opinion in Oklahoma Dept. of Environmental Quality v. EPA. An excerpt:
The Oklahoma Department of Environmental Quality petitions for review of a final rule promulgated by the Environmental Protection Agency establishing a federal implementation plan for the attainment of national air quality standards in “Indian country.” See Review of New Sources and Modifications in Indian Country, 76 Fed. Reg. 38,748 (2011) (codified at 40 C.F.R. pts. 49 and 51) (hereinafter Indian Country NSR Rule). Jurisdiction to implement the Clean Air Act lies initially in either a state or an Indian tribe. The EPA may in certain circumstances implement a federal program in Indian country, see 42 U.S.C. § 7601(d), but when it does so, in our view, it is subject to the same jurisdictional limitations as the tribe in whose shoes it stands. Because the EPA requires a tribe to show it has jurisdiction before regulating Indian country outside a reservation, yet made no demonstration of tribal jurisdiction before itself regulating those areas, we hold the agency was without authority to displace Oklahoma’s state implementation plan in non-reservation Indian country. We therefore grant the petition for review and vacate the Indian Country NSR Rule with respect to non-reservation lands.
Briefs are here.