db_fetch_array() fetches only one row from data table, even though I am expecting multiple results
Every time I have to take results outs of a database query result resource I'm scratching my head wondering why we don't have a way to return all rows without having to iterate over the object.  Perhaps there's a method out there in cTools or Drupal 7 but until a better answer is presented:

$results = db_query("SELECT nid from {og}");
while($row = db_fetch_array($results)) { 
  $group_node_nids[] = $row['nid'];
}