Returnpath has some very neat stats on how many people actually trust the "Unsubscribe" link, and how many people prefer to click "this is spam" instead. Best takeaway (IMHO):
"Make your email interesting and relevant! Subscribers in our survey report that the only factor growing in importance this year for why they open one email over another is "prior value" – they already received an email from you that was relevant. In fact, prior value with email falls just behind brand name in importance, and is more than twice as important as a discount offer."