Once you configure this, you do NOT want to change it; not ever. In fact, it is a VERY good idea to keep this backed up in a safe place, just in case you need to move your site, or re-install s2Member in the future. Some of the sensitive data that s2Member stores, will be encrypted with this Key. If you change it, that data can no longer be read, even by s2Member itself.
Cool ... so, what's the right way to back it up / restore it? I'd like to get this solid before I go live. Thanks!