From the Wikipedia Article.
Former Seattle Police Chief Gil Kerlikowske had this to say to his staff about questions regarding nude participants in the 2008 Gay Pride Parade:
Washington's indecent exposure law makes it a misdemeanor to 'make an open and obscene exposure of his or her person or the person of another knowing that such conduct is likely to cause reasonable affront or alarm.' In other words, there must be lewd or obscene behavior for an officer to take action. In order for the police to make an arrest, we must have witnesses currently in the public place where the nudity is occurring [sic] who must make a complaint. These witnesses must be willing to appear in court. Also in order to prosecute, the burden is on the government to prove that the offender was knowingly aware that their conduct created alarm and offense of others.
That pretty much makes minding your own business while naked legal. You have to be doing something obscene (probably a sexual act), and they have to be able to prove you knew someone else would be offended by your obscene act. Not going to happen while washing your car or riding your bike.
Note also that "Naked Gardening Day" started in Seattle.
But still most people won't go naked without company.
