Allright, coded the update function after my diner. When a customer updates his or her information on the "my account" page, this change will be synced with your Mail Chimp list. Here's how I dit it:

In mailchimp_functions.php from this great module I added my own function at the end of the file. Note that I added a field for Firstname and Lastname. You can do that, but if you don't want to just delete everything with firstname and lastname in it:

PHP Code:
//Mailchimp update function: update customers data to the mailchimp list.
 
function mailchimp_update($email_address_old$email_address$firstname$lastname$email_format) {
    include_once(
DIR_WS_CLASSES "MCAPI.class.php");
    
$api = new MCAPI(BOX_MAILCHIMP_NEWSLETTER_API_KEY);
    
$merge_vars = array('FNAME'=>$firstname'LNAME'=>$lastname'EMAIL'=>$email_address);
    if (
$email_format == 'TEXT') {
       
$format 'text'
    } else {
       
$format 'html'
    }

    
$list_id BOX_MAILCHIMP_NEWSLETTER_ID
    
$retval $api->listUpdateMember($list_id$email_address_old$merge_vars$format);
    if (
$api->errorCode){
        
$errorMessage "Unable to update member info with listUpdateMember()!\n" 
           
"\tCode=".$api->errorCode."\n" .
           
"\tMsg=".$api->errorMessage."\n";
          
$file DIR_FS_SQL_CACHE '/' 'MailChimp.log';
          if (
$fp = @fopen($file'a')) {
            
fwrite($fp$errorMessage);
            
fclose($fp);
          }
    }
 } 
In htdocs/includes/templates/YOURTEMPLATE/templates/tpl_account_edit_default.php you must make an invisible field to be able to pass the old email address (email_address_old) to the mailchimp_update() function. I've placed this line below the entry for the e-mail address, but you can paste in anywhere as long as it is within the form:

PHP Code:
<?php echo zen_draw_hidden_field('email_address_old'$account->fields['customers_email_address'], 'id="email-address-old"'?>
At last you must make a call to the function. When the form is submitted it will use /includes/modules/pages/account_edit/header_php.php to make all the magic happen. Paste the following around line 154 above the line:
$zco_notifier->notify('NOTIFY_HEADER_ACCOUNT_EDIT_UPDATES_COMPLETE');

PHP Code:
mailchimp_update($email_address_old$email_address$firstname$lastname$email_format); 
That's all! Now your mailchimp list will be updated as soon as your customer edits his or her information! Next thing to do is to make a sync for the "Subscribe or unsubscribe from newsletters." page.