It's really simple right now. We add a List-Unsubscribe header to each email which in supporting email clients (like gmail) adds an unsubscribe button to the report spam button that's always there. If the recipient unsubscribes in that way, we treat it as an unsubscribe. The more traditional subscription model isn't supported by Mandrill at this time. You'd need to build your own or use MailChimp for that. I recommend MailChimp, it's pretty good at handling subscription-based email like that ;)

It seems as if there are two sides of the fence with supporting unsubscribes from transaction emails.

Do you have any input regarding this?

Well, supporting unsubscribes in transactional emails can sometimes make sense, like with notification or confirmation emails where the user might want the option to not receive them. With others, like receipts or signup confirmations it makes less sense. For now, we presume that you're going to do the right thing and implement unsubscribes if it makes sense for the emails you're sending. We've got a feature coming to make that process easier, but it's still going to be a decision that the sender is going to need to make.