This is very similar to Massachusetts' Goodridge case. The challenge to state laws forbidding SSM in Olympia is to state law only. The court's ruling cannot go federal.
Note: The Goodridge link opens a PDF.
As Wyatt Buchanan's article notes:
The two Washington state cases explore two major issues being debated across the nation: whether marriage is a right and how the government can allow straight people to marry but not gays or lesbians.The outcome options are also similar to Massachusetts'. The court can decide:
"The cases present constitutional issues that judges haven't thought about a great deal yet," said Matt Coles, director of the American Civil Liberties Union's Lesbian and Gay Rights Project. "It's not just the narrow issue of marriage but how you think about laws that discriminate against gay people under the equal protection clause, how you think about what a fundamental right is.
- Uphold one-man/one-woman marriage.
- Overturn existing law and order authorization of SSM.
- Declare the law unconstitutional and turn the resolution back to the legislature.