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I just glanced at it, and found this in modules/newbb/viewforum.php:
$sortsince = !empty($HTTP_GET_VARS['sortsince']) ? intval($HTTP_GET_VARS['sortsince']) : 100;
$sel_since_array = array(1, 2, 5, 10, 20, 30, 40, 60, 75, 100);
...
$forum_selection_since .= '<option value="1000"'.(($sortsince == 1000) ? ' selected="selected"' : '').'>'.sprintf(_MD_BEGINNING,1000).'</option>';
The value 1000 appears to represent "From the beginning", so you could change the ": 100" to ": 1000".
I'm not an expert on the newbb module, so don't take this as the definitive answer.
