"Cross-site scripting (XSS) vulnerability in sidebar.php in WordPress, when
custom 404 pages that call get_sidebar are used, allows remote attackers to
inject arbitrary web script or HTML via the query string (PHP_SELF), a different
vulnerability than CVE-2007-1622."
Upstream doesn't seem to have a patch for this as of yet (May 17 2007).
wordpress-2.2.1-1.fc7 has been pushed to the Fedora 7 stable repository. If problems still persist, please make note of it in this bug report.