Index: wp-content/themes/twentyeleven/style.css
===================================================================
--- wp-content/themes/twentyeleven/style.css	(revision 21603)
+++ wp-content/themes/twentyeleven/style.css	(working copy)
@@ -969,6 +969,11 @@
 	max-width: 100%;
 }
 
+/* Make sure Twitter embeds are not fixed width */
+.entry-content .twitter-tweet-rendered {
+	max-width: 100%!important;
+}
+
 /* Password Protected Posts */
 .post-password-required .entry-header .comments-link {
 	margin: 1.625em 0 0;