From today's The Ethicist in the New York Times:
I am a fund-raiser at a not-for-profit. Part of my job is building up a list of people we can solicit financial support from. Last summer we worked on a voter-registration drive. My co-workers suggested that we add the names from registration forms to our database, pointing out that they are public information, and people can opt out of our list. Ethical? Anonymous, Tucson
Junk mail is in the eye of the beholder, like conjunctivitis; you should not contribute to the spread of either. Even if you legally obtain public information and use it for only a worthy cause, you should get permission before adding anyone to your mailing list. It's a matter of manners as much as morals. Besides, you'll do your cause little good if you vex potential supporters by bombarding them with unwanted mail.
Worse still, no matter how fastidious your organization, mailing lists have a way of escaping and snaking off to other, less scrupulous, mailers where they proliferate wildly, the kudzu of postbox and cyberspace. To avoid fertilizing this noxious underbrush, you should invite people to opt in to your database, not just allow them to opt out.