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I've moved my mailchimp list registration form onto the main user account page as per http://drupal.org/node/1780942 .
The problem now is, every time the user saves their profile, they get the message "You have updated your settings in list x" even when they have not changed their list subscriptions at all, which is confusing to users.
I think mailchimp_update_user needs to be patched to only show the update message if the user actually changes their mailchimp subscription settings.