As mentioned below, giving spamm'ers a way to test a block of (presumably random/dictionary-generated addresses) for valid subsets isn't a very good idea.
It would be nice if everyone that actually allows unsubscription would set the correct headers. In my experience, only traditional mailing-lists do, and all the stuff you really want to unsubscribe to reinvent a new interface over http(s). Which isn't great if you're in an email reader, not a web browser.
If people could get their act together and actually supply an accurate text/plain part to all the "fancy" html crap, that'd be great too.
It always makes me sad when companies realize that they've not got an interesting enough offer that stating it in a short concise text is going to get any conversions, an then assume that if they format it so it's unreadable on a small screen, add a lot of images that'll be blocked for privacy reasons, their offer will be more enticing.
Makes me even more sad that it (apparently) works.