Rich Brooks explores the question: when is it OK to take email addresses you've collected and add them to your email marketing campaign?. In his blog entry he explores several definitions of spam, and comes to some conclusions:

• There's a difference between an email newsletter and email blasts...just don't ask me why.
• Selling someone something is not the same as implied consent.
• A small quality list beats a large bought list every day of the week.
• Some people will tag your legitimate emails as spam rather than unsubscribe because they are just that lazy!
• You should be supplementing your email newsletters with a blog.