You can find the opinion in Wash. ex rel. Pub. Disclosure Comm’n v. Wash. Educ. Ass’n, Wash., No. 74268-5 here (there is a dissent, but I can’t find it on Findlaw and the Washington state court website appears to be down). (Thanks to Steven Sholk for the pointer.) A snippet:

In sum, RCW 42.17.760 regulates the relationship between the union and agency fee payers with regard to political expression. Therefore, we apply the framework set forth in Boy Scouts to determine whether sec.760 violates the union’s right of expressive association. The union engages in expressive activity and RCW 42.17.760’s opt-in requirement significantly burdens the union’s association with agency fee payers with regard to its political speech. Accepting the argument that protection of dissenters’ rights is a compelling state interest, the opt-out procedure is a less restrictive constitutionally permissible alternative. RCW 42.17.760’s opt-in procedure is not narrowly tailored to advance the State’s interest in protecting dissenters’ rights, and thus, the statute is unconstitutional.

The court distinguished Mich. State AFL-CIO v. Miller, 103 F.3d 1240 (6th Cir. 1997)). For more information on this issue, see the Lowenstein and Hasen casebook at 822-23.