Return Path has some good advice on a topic many email marketers get wrong: the unsubscribe process.
Yes, someone who's unsubscribing from your list is leaving and is unlikely to be a future customer... but you should still make the process as pleasant as possible. If you do, they might come back.
(And if you don't, you might wind up violating the CAN-SPAM law.)
This should be very simple, but often it doesn't work very well. I recommend checking your process against the five best practices that Return Path lists. There's no excuse for getting this one wrong - nothing they recommend is difficult.
I think I should forward this on to the nice marketing people at AT&T, since they have such trouble with the whole subject!