I had never heard of IPTC before (thanks for teaching me), but apparently it contains an abundance of fields. Among other a number pertaining to contact information, including multiple email adresses. That contact information is however supposed to point to the creator of the photo, There is also a field called "keywords" which seems more aimed towards what we call tags...

http://iptc.cms.apa.at/std/photometa...00907%29_1.pdf

The list of software supporting IPTC is however less than impressive. I couldn't find a corresponding hardware list but suppose the chances of anything Nokia or maemo supporting it are slim..?