I think we're going to have to switch the content type for the ajax scripts to UTF-8. This seems to fix it, but needs testing.
If you'd like to test on your install just change scripts/previewpost.inc.php. Line 79 looks like this:
header("Content-Type: text/html; charset={$ubbt_lang['CHARSET']}");
Change that, to this:
header("Content-Type: text/html; charset=UTF-8");