Over on the election law listserv, Rick Pildes argues here that we should read “Larios as holding that systematic partisan gerrymandering is an impremissible purpose under the Equal Protection Clause, pure and simple.” Rick further reads into the summary affirmance a view of Vieth at odds with what the Court actually did in the case a mere two months ago. Rick writes: “What does this suggest about Vieth? Most obviously, if partisan gerrymandering is not a legitimate purpose or rational basis here, it is also not one in the normal context in which plaintiffs bring affirmative challenges to districting plans. To the extent Vieth left this unclear, it now seems that most of the Court takes systematic partisan gerrymandering to be not just illegitimate and unconstitutional, but a serious constitutional problem; the difficulty in the Vieth context really is one of managable remedies, not a debate about whether partisan gerrymandering is unconstitutional. ”
I think that Rick reads too much into the summary affirmance. (And J.J. Gass agrees: see here, noting that if Rick is right, the Texas case should be reversed rather easily.)
In Illinois Elections Board v. Socialist Workers Party, 440 U.S. 173 (1979), the Court explained the reach of a summary affirmance (internal citations omitted):

[T]he precedential effect of a summary affirmance can extend no farther than “the precise issues presented and necessarily decided by those actions.” A summary disposition affirms only the judgment of the court below, and no more may be read into our action than was essential to sustain that judgment. Questions which “merely lurk in the record” are not resolved, and no resolution of them may be inferred.

In this case, the only issue necessarily decided here was that the 10% deviation was too much under the facts of the case. The Court may now simply agree, consistent with other lower court cases as well, such as the 4th Circuit’s Daly v. Hunt, that the state needs to come up with some good reasons for deviating from perfect equality even for state and local districts.
If Rick is right about the meaning of the summary affirmance, we would have expected the Chief Justices and Justices O’Connor and Thomas to joint Justice Scalia in dissent. Their failure to do so suggests that they do not see Larios as implicitly overruling Vieth, at least in those cases where there is a manageable rule to control partisan gerrymandering. (Of course, while one person, one vote is a manageable rule, I don’t concede that it is a manageable rule to control partisan gerrymandering).
UPDATE: Rick Pildes responds here.