multiple values

I've solved this need by using templates : views-data-export-xml-body.tpl.php and template.php.

views-data-export-xml-body.tpl.php

This is a quick fix to skip array tag by not printing it when encountered :

<?php foreach ($themed_rows as $count => $row): ?>
  <<?php print $item_node; ?>>
<?php foreach ($row as $field => $content): ?>
<?php if (isset($content) && $content!=""): ?>
<?php
      // Suppress Array tag Array for multiple fields
      // CAUTION : the name of the tag to hide depend from Tranform Type setting in Views. In this case : camelCase
      // This is a quick fix after all !
      if ($xml_tag[$field] == "Array"): ?>
   <?php print $content; ?> 
<?php else: ?>
    <<?php print $xml_tag[$field]; ?>><?php print $content; ?></<?php print $xml_tag[$field]; ?>>  
<?php endif; ?>
<?php endif; ?>
<?php endforeach; ?>
  </<?php print $item_node; ?>>
<?php endforeach; ?>

template.php

Override the function theme_content_view_multiple_field() in your theme's template.php (from http://drupal.org/node/556232#comment-1957166). Don't forget to empty cache !

function mytheme_content_view_multiple_field($items, $field, $values) {
  if ($field['field_name'] != 'field_my_tuned_multivalue_field') {
    // If this is not my tuned field, then use original theme function directly.
    return theme_content_view_multiple_field($items, $field, $values);
  }

  // Separate item values with my favorite separator.
  $separator = ', ';
  $array = array();
  foreach ($items as $item) {
    if (!empty($item) || $item == '0') {
      $array[] = $item;
    }
  }
  return '<div class="field-item">'. implode($separator, $array) .'</div>';
}

At least, I did not give enough time to find why the array tag appear.