If you're looking at maybe changing the header (above the breadcrumb, navigation bar) you could use the Display Options -> General -> HTML Includes section. In the Default Header, you could do something like:

Code
<?php
if ($user['USER_ID']) {

   echo <<<USER_IS_LOGGED_IN

This will display to a user that is logged in.

USER_IS_LOGGED_IN;

} else {

   echo <<<REGISTER_PRETTY_PLEASE

This will display to a user that is not logged in.

REGISTER_PRETTY_PLEASE;

} // end if
?>