When Does Signing Up Become 'Opting In?'

AmyVernon writes "This piece from RWW got me thinking about whether, when you sign up for access to a site, you're actually signing up to get a slew of email spam from them. The single opt-in is still really popular, which I've noticed because I often check the box indicating I don't want further emails from a company or publisher. I always assume that giving my actual email address means I'm going to get spam-type emails from whomever. It still surprises me that most people don't. But it does raise a good question: Shouldn't you be able to sign up for something without automatically being signed up for a never-ending stream of 'updates?'"