Something I've had bookedmarked for a little while is the following (yet untested),
Source LINK
//add our action
add_action( 'save_post', 'my_save_post', 11, 2 );
function my_save_post($post_id, $post){
//if it is just a revision don't worry about it
if (wp_is_post_revision($post_id))
return false;
//if the post is an auto-draft we don't want to do anything either
if($post->post_status != 'auto-draft' ){
// unhook this function so it doesn't loop infinitely
remove_action('save_post', 'my_save_post' );
//this is where it happens -- update the post and change the post_name/slug to the post_title
wp_update_post(array('ID' => $post_id, 'post_name' => str_replace(' ', '-', $_POST['post_title'])));
//re-hook this function
add_action('save_post', 'my_save_post' );
}
}