In this case, you get an e-mail that says: "Periodically we'll drop you a line if we have something interesting to share." (There's also an unsubscribe link.) They'll get to build up an incredible list with this.
I'm happy to see 37signals using this approach as it helps legitimize the technique of building a list using free content without disclosing a subscription up front. Till now I've avoided this technique but I imagine it could convert like crazy and build a good list if done right.