Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 17333)
+++ wp-includes/formatting.php	(working copy)
@@ -188,6 +188,7 @@
 	$allblocks = '(?:table|thead|tfoot|caption|col|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|option|form|map|area|blockquote|address|math|style|input|p|h[1-6]|hr|fieldset|legend|section|article|aside|hgroup|header|footer|nav|figure|figcaption|details|menu|summary)';
 	$pee = preg_replace('!(<' . $allblocks . '[^>]*>)!', "\n$1", $pee);
 	$pee = preg_replace('!(</' . $allblocks . '>)!', "$1\n\n", $pee);
+	$pee = str_replace('<hr />', "\n\n<hr />\n\n", $pee);	// hr's are block-level and self-closing
 	$pee = str_replace(array("\r\n", "\r"), "\n", $pee); // cross-platform newlines
 	if ( strpos($pee, '<object') !== false ) {
 		$pee = preg_replace('|\s*<param([^>]*)>\s*|', "<param$1>", $pee); // no pee inside object/embed
@@ -207,6 +208,7 @@
 	$pee = str_replace('</blockquote></p>', '</p></blockquote>', $pee);
 	$pee = preg_replace('!<p>\s*(</?' . $allblocks . '[^>]*>)!', "$1", $pee);
 	$pee = preg_replace('!(</?' . $allblocks . '[^>]*>)\s*</p>!', "$1", $pee);
+
 	if ($br) {
 		$pee = preg_replace_callback('/<(script|style).*?<\/\\1>/s', '_autop_newline_preservation_helper', $pee);
 		$pee = preg_replace('|(?<!<br />)\s*\n|', "<br />\n", $pee); // optionally make line breaks