Validate based on existing CCK field value?
You should be able to use hook_nodeapi for this!

In this example I'm assuming the node type is blah

In a custom module just write something along the lines of:

function mymodule_nodeapi(&amp;$node, $op, $a3 = NULL, $a4 = NULL) {
  if ($op == 'validate' &amp;&amp; $node-&gt;type == 'blah') {
    $sql = db_query("SELECT bl.field_address 
           FROM {content_type_blah} bl
           WHERE bl.field_address = '%s'", $node-&gt;field_address)

    if ($sql != '') {
        form_set_error($node-&gt;field_address, t('This address is already being used'));
    }
  }
}


That way, before a node is saved (during the validation) it runs an SQL query to see if the address field is duplicated anywhere else on any other node with the type blah.  If it is, then it throws an error on the form field!  You will need to tweak the query dependant on your table names etc.

I'm only writing that code from the top of my head, and I've not tested it (sorry) but it should definately be a starting point!  I may have the $node->field_address part wrong!

If all else fails, put a die(print_r($node)); after the if ($op == 'validate'... line and you can get the right part from that!

Hope this helps :)

References:

http://api.drupal.org/api/drupal/developer--hooks--core.php/function/hook_nodeapi/5
http://api.drupal.org/api/drupal/includes--form.inc/function/form_set_error/6