new page sidebar-template by default

I would hook into the new_page action, and then use update_post_meta() to update the post meta value for _wp_page_template:

<?php
function wpse31124_set_page_default_template( $page_id ) {
    update_post_meta( $page_id, '_wp_page_template', 'sidebar-page.php' )
}
add_action( 'new_page', 'wpse31124_set_page_default_template' );
?>

You could hook into other actions, but they might stomp on your ability to use the default template. Using new_page will ensure that the callback only changes the page template meta data value when a new page is created initially.