Index: wp-content/themes/twentytwelve/style.css
===================================================================
--- wp-content/themes/twentytwelve/style.css	(revision 21733)
+++ wp-content/themes/twentytwelve/style.css	(working copy)
@@ -881,6 +881,9 @@
 	line-height: 1.846153846;
 	color: #777;
 }
+.single-author .entry-meta .by-author {
+	display: none;
+}
 
 
 /* =Archives
Index: wp-content/themes/twentytwelve/functions.php
===================================================================
--- wp-content/themes/twentytwelve/functions.php	(revision 21733)
+++ wp-content/themes/twentytwelve/functions.php	(working copy)
@@ -336,11 +336,11 @@
 
 	// Translators: 1 is category, 2 is tag, 3 is the date and 4 is the author's name.
 	if ( $tag_list ) {
-		$utility_text = __( 'This entry was posted in %1$s and tagged %2$s on %3$s by %4$s.', 'twentytwelve' );
+		$utility_text = __( 'This entry was posted in %1$s and tagged %2$s on %3$s<span class="by-author"> by %4$s</span>.', 'twentytwelve' );
 	} elseif ( $categories_list ) {
-		$utility_text = __( 'This entry was posted in %1$s on %3$s by %4$s.', 'twentytwelve' );
+		$utility_text = __( 'This entry was posted in %1$s on %3$s<span class="by-author"> by %4$s</span>.', 'twentytwelve' );
 	} else {
-		$utility_text = __( 'This entry was posted on %3$s by %4$s.', 'twentytwelve' );
+		$utility_text = __( 'This entry was posted on %3$s<span class="by-author"> by %4$s</span>.', 'twentytwelve' );
 	}
 
 	printf(
@@ -354,6 +354,21 @@
 endif;
 
 /**
+ * Adds a classe to the array of body classes.
+ * If the site has only one author with published posts.
+ *
+ * @since Twenty Twelve 1.0
+ */
+function twentytwelve_body_classes( $classes ) {
+
+	if ( function_exists( 'is_multi_author' ) && ! is_multi_author() )
+		$classes[] = 'single-author';
+
+	return $classes;
+}
+add_filter( 'body_class', 'twentytwelve_body_classes' );
+
+/**
  * Extends the default WordPress body class to denote:
  * 1. Using a full-width layout, when no active widgets in the sidebar
  *    or full-width template.