Generally, I wouldn't trust it unless it comes from a service you use.
For example, I might signup for service X and not notice the checkbox to opt-out of email marketing. If they start sending me email I would expect there to be an unsubscribe link, since it's required by law. I would expect them to follow the law since I wouldn't create an account on a non-trustworthy website. Furthermore, I gave them my email during the registration process, so they already know it's a valid address and don't need to wait for a click on a link to prove that.
The law also says that "it does not require e-mailers to get permission before they send marketing messages." Which means that even services you didn't sign up for are allowed to send you email. This makes it tricky since that service may or may not be legit. If it's a well known company I would say it's safe to click the unsubscribe link (make sure it's going to their domain though, since the email might be spoofed). If it's from some other company it's a tough call. Personally, I wouldn't click on it, but someone else might have better advice for you.
Another thing you need to consider is that spammers might add the unsubscribe link not only to prove that it's a real address, but also to confuse the Bayesian filter into thinking it's a non spam message.