I think you should remove this link. It is just not nice.
Yes, I know, 37signals is a bit more respectable than most of the people who want your email. Nevertheless they're signing you up for an email list without explicitly asking you -- the email that they send back specifically says, "Periodically we'll drop you a line if we have something interesting to share." I did not check any box which agreed to that, and I shouldn't have to unsubscribe from things I never subscribed to.
I'm just surprised that 37signals are using techniques of "six-pack in a week"-kind of sites.
Gmail hides the spam from me and it's quick filter wizard helps me hide anything else I want to.
You people are stuck on the wrong side of an A/B experiment. Tough luck~