Member since date is being updated on import but we dont want it to!
Confirmed this is happening on Drupal 6 and CiviCRM 4.4.14. In file: drupal\sites\all\modules\civicrm\CRM\Member\Import\Parser\Membership.php

line number 268 reads:-
 
     //assign join date equal to start date if join date is not provided
     if (!CRM_Utils_Array::value('join_date', $params) &amp;&amp;
        CRM_Utils_Array::value('membership_start_date', $params)
      ) {
        $params['join_date'] = $params['membership_start_date'];
      }


You could try commenting out this line if it's not possible to calculate the join_date from your data.