The Court’s order is here; Lyle Denniston’s early analysis is here.
I must say that I am surprised by the cert. grant. Here I predicted a cert denial, noting that “I was wrong about the last ‘parties’ case the Court took, Clingman v. Beaver,” as well. Here is what I wrote when the Ninth Circuit issued its unanimous opinion striking down Washington’s law:

This unanimous opinion in Washington State Republican Party v. State of Washington is a very important one, clarifying the reach of some dictum in the Supreme Court’s 2000 opinion in California Democratic Party v. Jones. Jones held that a blanket primary (where all voters could vote for any party candidate, regardless of the voters’ party affiliation) violated the First Amendment associational rights of political parties that objected to this form of primary. Jones in dicta held that a truly nonpartisan primary, with the top two candidates going on to a run-off, would not violate the parties’ First Amendment rights.
In response to Jones (which caused Washington State’s blanket primary to be struck down), voters passed a measure imposing a top two primary, but allowing candidates to use their party “preferences” on the ballot. Today the Ninth Circuit held that this form of primary violated that First Amendment rights of parties under Jones, because the use of the party label still forced association on parties that did not want it.
I was involved in the drafting of Proposition 62 in California, which—had it passed—would have imposed a similar system for California primaries. But there was a crucial difference. Parties that objected to the use of party labels would have the option of not using those labels on the ballot. (It would be a global choice—a party could not give Candidate A, but not Candidate B, the right to use the party label.) The Ninth Circuit’s opinion does not opine on whether or not such a system would survive a challenge under Jones—a question the court may eventually face if Washington voters try again with such a system.

Because the Washington system included such party labels, I expected a majority of the Court to agree with the Ninth Circuit that the Washington system was functionally indistinguishable from the California system struck down in Jones. That is, if a party objected to the blanket primary and could opt it, it should have the right to opt out of the Washington top two as well.
So the cert. grant might mean that enough Justices have doubts that the Washington system is indistinguishable from the California system. Or it might be—and here’s a trend I see perhaps developing with the grant of Lopez-Torres as well—that the Court simply finds these election law cases interesting, and enticing to take even when there are not high stakes involved (as in the major campaign finance cases, redistricting cases, or Bush v. Gore).