In this blog post, Al Iverson states that confirmed opt-in and double opt-in both mean the following and only the following:
"A potential recipient submits an email address at a web page. This triggers a confirmation request email. No further emails are sent to the end recipient until and unless they take positive action to confirm the subscription in response to this confirmation request email. That means the person who received the confirmation message has to click on a link (or respond to a token, but I prefer the link method) to confirm the subscription. If they didn't do that, then you don't consider them opt-in, and you don't email them further.
If somebody uses the term confirmed opt-in to mean filling out a web form and receiving an email saying “Your subscription is confirmed. If this is incorrect, click here,” then they are mistaken. This isn't confirmed opt-in or double opt-in. It's a signup form with a welcome message. The welcome message lets the recipient opt-out if necessary, and that's great – but it's not confirming anything as far as the opt-in police (ISPs, blacklists, etc.) are concerned. I see a lot of confusion surrounding this and it's important to remember the following: It's not confirmed opt-in or double opt-in unless the recipient has to take that active step of clicking on a YES link or taking some other YES-affirming action."