I have a dilemma kind of like what Jason blogged about a while back. He was looking for some software that would do tagging of photos that met a few specific needs. I have similar needs and I decided to go with digiKam partly as a result of the research he did. So I've been tagging some of my photos in digiKam and I can do some helpful searches thanks to this effort. Now I'm running in to a limit of tags though. Tags are handy because they're a quick way to say "my mom is in this photo" or "some of my family members are in this photo." The quick and simple approach of using a first name or a personal nickname really doesn't work well for a complete collection though. I have thousands upon thousands of photos that I've shot over the past few years and I'd love to embed some metadata in them that lets me group them in useful ways.
As an aside, of course I don't really use my originals for this right now. I keep a read-only copy that remains as it came from the camera and I have another copy that I do things to with software.
My first try was to just say I'll use first names of people but that broke down right away. Candace has friends with the same names as some of my siblings. So now I've been trying to think of other options. I know there's a method of embedding RDF in EXIF but that sounds like overkill and I don't know if I can convince digiKam to do that. Another easy option would be to use a unique key like email address. That would be pretty convenient and if I ever did move to RDF I could write some scripts to do the conversion for me.
Norman Walsh seems to have some experience with this but the learning curve seems pretty steep. Maybe it's time to build some of this functionality into a plugin for digiKam...