Vik Amar sends along the following reactions to my earlier query:

Rick has rightfully suggested in various fora that the question when the Constitution’s mention of a state “legislature” constitutes a reference to a specific body of state government — as distinguished from a reference to a legislative process — is one that seems to recur with increasing frequency since Bush v. Gore. As I have written, see 35 Hastings Con. L. Q. at 735, the Court’s cases seem more inclined to disallow interference with a state legislature’s prerogatives when that interference comes from another branch of government rather than from the people themselves. (For example, as Rick and I have both pointed out, the three members of the Court who joined Chief Justice Rehnquist’s concurrence in Bush v. Gore — and its strong reading of the word “legislature” in Article II — dissented from the Court’s denial of certiorari in Colorado Gen. Assembly v. Salazar, 541 U.S. 1093 (2004) (cert. denied), indicating their discomfort under Article I, Section 4, which also empowers state “legislatures,” with state judicial involvement in Congressional district line drawing. But, notably, their dissent from the denial of certiorari in Salazar intimated that popular, as opposed to judicial, involvement would be less constitutionally troubling.)
So, in the context of the Seventeenth Amendment, the specific textual division of power between state legislatures and (when it comes to filling vacancies) state Governors seems important to respect. But the people asserting their will as the super-legislature of the state ought not to be as problematic. This seems all the more true given that the thrust of the Seventeenth Amendment was empowerment of the statewide electorate at the expense of the power of the elected state legislature. See generally Vikram David Amar, “Are Statutes Constraining Gubernatorial Power to Make Temporary Appointments to the United States Senate Constitutional Under the Seventeenth Amendment,” 35 Hast. Con. L. Q. 724 (2008).

Meanwhile, Stevens has said he’s not resigning, so all of this may be just academic speculation.