blahhhblahh.php

<?php

// HINDSIGHT: Don't use presave. You'd need save (right, that exists?)
function mymodule_commerce_customer_profile_save($profile) {
  if (isset($profile->my_custom_boolean)) {
    // Okay so the field was set, or exists on this bundle.
    if (!empty($profile->my_custom_boolean[LANGUAGE_NONE][0])) {
      // Okay, we have a value (double check this var)
      variable_set('commerce_addressbook_defaults', $profile->id);
    }
  }
}