Index: wp-includes/functions-formatting.php
===================================================================
--- wp-includes/functions-formatting.php	(revision 2557)
+++ wp-includes/functions-formatting.php	(working copy)
@@ -392,6 +392,8 @@
 	}
 
 	$tagstack = array(); $stacksize = 0; $tagqueue = ''; $newtext = '';
+	$single_tags = array('br', 'hr', 'img', 'input'); //Known single-entity/self-closing tags
+	$nestable_tags = array('blockquote', 'div', 'span'); //Tags that can be immediately nested within themselves
 
 	# WP bug fix for comments - in case you REALLY meant to type '< !--'
 	$text = str_replace('< !--', '<    !--', $text);
@@ -442,11 +444,11 @@
 			if((substr($regex[2],-1) == '/') || ($tag == '')) {
 			}
 			// ElseIf it's a known single-entity tag but it doesn't close itself, do so
-			elseif ($tag == 'br' || $tag == 'img' || $tag == 'hr' || $tag == 'input') {
+			elseif ( in_array($tag, $single_tags) ) {
 				$regex[2] .= '/';
 			} else {	// Push the tag onto the stack
 				// If the top of the stack is the same as the tag we want to push, close previous tag
-				if (($stacksize > 0) && ($tag != 'div') && ($tagstack[$stacksize - 1] == $tag)) {
+				if (($stacksize > 0) && !in_array($tag, $nestable_tags) && ($tagstack[$stacksize - 1] == $tag)) {
 					$tagqueue = '</' . array_pop ($tagstack) . '>';
 					$stacksize--;
 				}