a way to show/hide a field on the subscription form (with anon/auth distinction)

use hook_form_alter from the Drupal api

Great idea, but I think it's a fairly fringe use case, and the interface is already very busy, this would make it more so. I'll consider a patch, but am hesitant to include this. Indeed, it's exactly what hook_form_alter() is for.

Agreed on leaving it postponed. If you want to submit a patch, for that matter, I'd be happy to consider for inclusion. Thanks.

Often I want to sync user profile info once they complete the registration process, but in the sidebar, only the email is required. (I'd prefer to not use CSS to hide elements). Hook form alter sounds like a temp fix to me.

You can easily create a custom module using form_alter hook to hide the fields you don't want in a form,
find out the form ID, find out the fields in the array you want to hide,
you can use dsm($form) if you are using devel module, or simply use print_r($form) to print out the array.

The following code sample is to hide the first name and last name fields in the subscription block.

<?php
  function YOURMODULENAME_form_alter(&$form, $form_state, $form_id) {
    switch ($form_id) {
      case 'MAILCHIMP FORM ID':
		unset($form['mailchimp_lists']['RAMDOM NUMBER']['FNAME']);
		unset($form['mailchimp_lists']['RAMDOM BUMBER']['LNAME']);
        break;
    }
  }
?>

actually its

  function YOURMODULENAME_form_alter(&$form, $form_state, $form_id) {
    switch ($form_id) {
      case 'MAILCHIMP FORM ID':
        unset($form['mailchimp_lists']['RAMDOM NUMBER']['mergevars']['FNAME']);
        unset($form['mailchimp_lists']['RAMDOM BUMBER']['mergevars']['LNAME']);
        break;
    }
  }

mm, actually that doesn't work.

lol, seems i forgot to reference the $form in the function.

function mailchimp_hacks_form_alter(&$form, $form_state, $form_id) {
    if ($form_id === 'mailchimp_lists_user_subscribe_form_sign_up_for_our_newsletter') {
        $form['mailchimp_lists']['mailchimp_sign_up_for_our_newsletter']['mergevars']['FNAME']['#access'] = false;
        //dsm($form);
    }
}

This feature is now available in 2.11.