That single opt-in hack for MailChimp is not exclusive to multisite installations.
You should tell the user somewhere that he’ll be getting your emails, or I believe it may could cause you trouble with MailChimp. You can mention it during registration and also in the New User email. [hilite path]Dashboard -› s2Member® -› General Options -› Email Configuration -> New User[/hilite]
Not letting users set their password on registration, but having WordPress generate one and email it to them in the New User email, requiring them to go open the email to then login, could be considered a double opt-in. It also works great to confirm the email address is valid. [hilite path]Dashboard -› s2Member® -› General Options -› Registration/Profile Fields -> Allow Custom Passwords[/hilite]
The user would be added to the MailChimp list on registration, not on login, so it’s not like he’d activate the account in some way before being added to the list, accounts are active the moment they’re created in WordPress, unless some other plugin alters this.
I hope that clarifies things a bit. :)