In the "Advanced Search", the contextual help about how to use the "%" wildcard returns char data without an encoding. For example, in French, this means that accentued characters show strange since the text is in UTF-8 but most browsers default on ISO-8859-1.
I fixed my installation with the following modification:
return "<div class='crm-help'>" . mb_convert_encoding($text,'HTML-ENTITIES', 'UTF-8') . "</div>";
in file CRM/Core/Smarty/plugins/block.htxt.php, line 49.
Altough the headers of the ajax reply showed (in Firebug) that the web server was responding: «Content-Type: text/html; charset=iso-8859-1», so this could also be the source of the problem. However, I couldn't find in the Civicrm source where the content-type headers were generated. Is it possible that ajax.php is not replying any Content-Type header at all?
mathieu, aka "bgm" on #civicrm