You could filter the_content
:
add_action( 'post_updated', 'wpse105892_add_message', 10 );
function wpse105892_add_message() {
add_filter( 'the_content', 'wpse105892_display_message' );
}
function wpse105892_display_message( $content ) {
// remove the action once it's run
remove_action( 'post_updated', 'wpse105892_add_message', 11 );
$content = "<div class="your-message">You did it!</div>\n\n" . $content;
return $content;
}