Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 38339)
+++ src/wp-includes/formatting.php	(working copy)
@@ -474,6 +474,11 @@
 	// Add a double line break below block-level closing tags.
 	$pee = preg_replace('!(</' . $allblocks . '>)!', "$1\n\n", $pee);
 
+	// hr's are block-level and self-closing
+	$pee = preg_replace('!(<hr\s?/?>)!', "\n\n$1\n\n", $pee );
+	//$pee = str_replace( '<hr>', "\n\n<hr>\n\n", $pee );
+	//$pee = str_replace( '<hr />', "\n\n<hr />\n\n", $pee );
+
 	// Standardize newline characters to "\n".
 	$pee = str_replace(array("\r\n", "\r"), "\n", $pee);