GeoServer 1.6.4 has been rebuilt, this time based on Geotools 2.4.4. This patched release (known as 1.6.4b on SourceForge) fixes the following issue:
http://jira.codehaus.org/browse/GEOS-1942
If you have downloaded 1.6.4, please download it again.
http://geoserver.org/display/GEOS/GeoServer+1.6.4
Thanks,
Mike Pumphrey
Outreach Engineer
The Open Planning Project