Aloha mc_jesse and all!
I propose that you use plugins_url() in place of WP_PLUGIN_URL within mailchimpSF_where_am_i().
Why? WP_PLUGIN_URL does not take into account whether the request uses SSL and always returns the Non-SSL URL. This triggers "Connection Partially Encrypted" browser warnings when a page is requested over https while the mailchimp.js is served over plain http.
More on plugins_url() Introduced in 2.6: