Beware of the crossing state lines thing wrt that not being federally legal thing. Granted, this perception is from your television shows, but damn, Crossing State Lines seems to be a big-ass crime, and though pot might be legal in both states, they might not allow import from one state to another .. and the actual boundary is probably considered federal jurisdiction. :P I think I've heard that some places might have booze laws like that (independent of whether a county/given jurisdiction is "dry" or not) - legal to buy and possess, but not for moving over state lines.

That being said, I wouldn't fly with it until the other Washington does something about it on the federal level. And if you do drive, check your lights often so there's no reason to stopcheck you. Or just go the "mail it" route, that's probably safest of all, if your government isn't so snoopy about mail.