Brad Taylor has a post on the official Gmail blog talking about the new unsubscribe option. There are two points I didn’t cover here yesterday.
you’ll only see the unsubscribe option for senders that we’re pretty sure are not spammers and will actually honor your unsubscribe request. We’re being pretty conservative about which senders to trust in the beginning; over time, we hope to offer the ability to unsubscribe from more email.
[…]If you want to unsubscribe without reporting the message as spam, click “show details” in the top-right corner of the message, then click “Unsubscribe from this sender.”
This should answer the bulk of the questions left in the comments yesterday.