Not sure if it's considered a bug, or if it's intentionally, but the Mail Preview in admin (Tools -> Send Email) is now displaying HTML code. Confirmed in new clean install with English only, and no text editor installed - just enabled support for HTML mails in config.

The change causing this is in line 253 in /admin/mail.php which was changed from:
Code:
<?php $html_preview = stripslashes($_POST['message_html']); echo (stristr($html_preview, '<br') ? $html_preview : nl2br($html_preview)); ?><hr /></td>
Into:
Code:
<?php $html_preview = zen_output_string_protected($_POST['message_html']); echo (stristr($html_preview, '<br') ? $html_preview : nl2br($html_preview)); ?><hr /></td>
Had some complaint about this from admin users, who didn't like it as they can't read HTML.