Windows Live Beta has implemented an Unsubscribe button based on the List-Unsubscribe header. It still is in beta so things may change and may want to cut them some slack if some issues are still being sorted out.
In Windows Live Beta, there is an area just above the message headers that show the Subject and who the messages is From where informational messages are displayed. Most messages display a "Report & Delete" link - clicking on this reports the message to Microsoft as spam and adds the sender's From address to the user's personal block list.
If the sender's From address is in the user's address book OR the sender is in the ReturnPath Sender Score Certified program, the List-Unsubscribe header will cause an "Unsubscribe" link to be displayed instead of "Report & Delete".
When "Unsubscribe" is clicked, a message is sent to the address specified in the List-Unsubscribe header (only mailto is supported at this time) and the sender's From address is added to the user's personal block list. I'm worried that this behavior will confuse users when they try to resubscribe later.