If you find yourself in a position where you need to update a multiple value CCK field by writing your own SQL, make sure you don’t forget to clear the cache!
cache_clear_all('content:'. $node->nid .':'. $node->vid, 'cache_content');
Not clearing the cache will result in your updates not being shown in the node details, despite the fact that the database was updated correctly.
See also http://drupal.org/node/133705#comment-625180.