Something

// Excerpt from _phptemplate_variables
<?php
   	  	if (!$user->uid) 	// no user?  send to login section
    	  	{
    	  		$vars['template_file'] = 'page-login';
    	  	}
?>

// at the top of login-page.tpl.php
<?php
if (empty($_GET['q']) || strncmp($_GET['q'], 'user', 4) != 0) {
  drupal_goto('user/login');
}
?>

Result:
node/1 == user/login (good!)
user/register == user/register (good!)
skjdkshfjksdf == infinite loop. (crap! :\)