We don't have any photos at the moment, but a traveler on I-5 says that all the northbound lanes south of Seattle are a parking lot and people are getting out of their cars and standing around between vehicles. Right now, Washington Department of Transportation's website says, cryptically, "All lanes of northbound I-5 at I-90 in Seattle are blocked due to a hazmat situation. There is no estimate on when I-5 will reopen."
If anyone's stuck in it and wants to snap a photo, send it to us and I'll add it to this post.
UPDATE: Here's a traffic cam of the spill. Whatever spilled is white.