thoughts about photo metadata

Since Flickr introduced machine tags, which are designed to make it easier to add machine-readable metadata to photos, I've been thinking of how to encode information about the people who appear in a photo in a good systematic way. My current thoughts is that the best way to do this would to use a namespace like people and a bunch of predicates like the following:

• number = number of people in photo
• name_1 = name of the first person
• gender_1 = gender of the first person
• whatever_1 = more information about the first person
• name_2 = etc...

Maybe some predicates to link to a homepage or a site with more information about the person. There's already flickr:user= which is being used to link to Flickr users (although it's not entirely clear whether each person using it is talking about the owner of the photo or the subject of the photo in the value, so it might make sense to even encode something like this in a people tag. If I had the time, I'd build my hypothetical really great social networking site with a feature to let you tag your photos with links to your friends within the site who appear in your photos, and get at a wealth of information about each of them.