Today my friend Stephen Sachs and I filed an amicus brief on behalf of ourselves and in support of neither party. In a nutshell we argue that Hall is mostly right, partly wrong, but that this case is a bad vehicle for dealing with any of the wrong parts and the case should be dismissed -- among other reasons because the Supreme Court itself may lack jurisdiction over the appeal.

And from the brief:

Hall correctly held that States lack a constitutional immunity in sister-state courts. The Founders viewed each State as immune from such suits under the common law and the law of nations. Before the Constitution's enactment, this was plainly not a constitutional right, and nothing in the Constitution changed that. Thus, Hallproperly rejected the argument that there is a "federal rule of law implicit in the Constitution that requires all of the states to adhere to the sovereign-immunity doctrine as it prevailed when the Constitution was adopted." 440 U.S. at 418.

A plain reading of the Constitution's text reveals no affirmative guarantee of sister-state immunity. Unless otherwise specified "in the plan of the convention," The Federalist No. 81, at 549 (Alexander Hamilton) (Jacob E. Cooke ed., 1961), the Constitution takes the States' sovereign rights as it finds them. The Founders left the rules of sister-state immunity precisely as they were: as ordinary rules of common law and the law of nations, to be enforced through ordinary channels.

...

Hall went too far, however, in denying that "the Constitution places any limit on the exercise of one State's power to authorize its courts to assert jurisdiction over another State," 440 U.S. at 421 (emphasis added), and in reducing sister-state immunity to a "matter of comity," id. at 425. In particular, Hall was likely wrong to assume that a State's abrogation of immunity in its own courts could be projected outward so as to bind other state and federal courts.

Even without a direct guarantee of sister-state immunity, the Constitution may well offer substantial indirect protection rooted in the law of judgments. Whatever a State's power to prescribe rules for its own courts, it cannot force other courts to take notice of "a jurisdiction which, according to the law of nations, [the] sovereign could not confer." Rose v. Himely, 8 U.S. (4 Cranch) 241, 276–77 (1808). Early federal and state courts routinely refused full faith and credit to state judgments that exceeded the jurisdictional limits imposed by the law of nations and the common law. These same principles would provide ample protection for States today, shielding them from suit in sister-state courts without inventing a novel constitutional rule.