Public IPTC forum on metadata for photos.
The IPTC maintains standards for metadata fields to be used with photographs: the "IPTC (meta)data" since 1994 (first introduced by Photoshop) and the "IPTC Core" (since 2005) and "IPTC Extension" (since 2008) used with Adobe's XMP technology (Photoshop 7.1 and higher).
Further the IPTC maintains controlled vocabularies for the "subject" of photos and for the "scene" of a photo.

This all and the future development of photo metadata standards may be discussed in this forum.
Specific (technical) issues with the "IPTC Core" standard may be discussed in the IPTC for XMP Yahoo forum http://tech.groups.yahoo.com/group/iptc4xmp/

The activities on this group are hosted and moderated by the IPTC.
Any member of this IPTC moderated Yahoo group must comply with the Intellectual Property Policy of the IPTC, available at www.iptc.org/goto/ipp. Any posting is assumed to be submitted under the conditions of this IPTC IP Policy.