Gmail offering unsubscribe option

This morning Lifehacker reported that Gmail was offering an option to unsubscribe from some legitimate email lists.
Gmail’s help pages say:

We don’t think you should be burdened with managing messages you don’t want to receive. We do our best to put messages in Spam when we’re pretty sure you won’t want or need them. But everyone has different preferences about the mail they want to see. You may not want to read any messages sent by a certain company or mailing list, while another Gmail user finds these same messages to be valuable.
To help solve this problem, we’re providing you with an unsubscribe tool for some messages. You’ll see the unsubscribe tool when you mark a message from particular types of mailing lists as spam. If the particular message is a misuse of a mailing list you like to receive, you can Report spam as usual. But if you never want to receive another message or newsletter from that list again, click Unsubscribe instead. We’ll send a request to the sender that your email address be removed from the list. It’s that simple!

This prompted a number of us to start testing Gmail to see if we could identify what Gmail was using in order to present the unsubscribe message to the end user. Many thanks to the folks who tested various things and reported back the results.
Conditions where the unsubscribe option is presented include

• The mail is authenticated
• The sender has a good reputation
• The email has a mailto: option in the List-Unsubscribe header
• The recipients marks the message as spam

This is a small step along the path to an ISP mediated unsubscribe button. As of yet, I don’t know if other ISPs will adopt this model. As well, I expect senders will not like this implementation as the ‘unsubscribe’ option is only presented if they user has marked the message as spam.
On the other hand, this could simply be Gmail’s attempt to implement a feedback loop without the overhead involved in actually managing a feedback loop.