Oh, and btw:

Right. In fact, popular web-services extract information like title, description and tags from exactly these fields when you upload an image that contains such info. And yes, there's no field I'd know of that is exclusively meant for including "the email address of the person depicted".

Doesn't matter much, though. One could do one of three things:

a) Use tags, as you proposed. The tag wouldn't read "joe@his.net", though, but "person:mail=joe@his.net". This way it will be easy to find for applications looking for it - and will still make sense to those who only take tags as non-structured words. (flickr did this before they had proper geotagging: You could prefix tags with something like "geo..." to give latitude/longitude in an plain-text field.)

b) Find yet another field that's usually not presented to humans by photo viewing software. (Tags, titles, descriptions, dates etc. are)
It's prettier (because we never don't get to see the cryptic "person:mail=joe@his.net") and has another advantage... which i'll discuss later.

c) Use XMP. XMP was an attempt by Adobe to bring the flexibility of RDF to image metadata. I hear there are architectural issues with it, but in general people say it's expandable - which means: It has a set of pre-defined fields, but you can add your own and give them a well-defined meaning. Adding existing FOAF-attributes to XMP-tags could work, because both are RDF-based. This would save us the work of defining anything Maemo-specific. We'd just use 2 existing standards that live within the same framework. (I'm a little worried though that I don't find any examples of FOAF in XMP via google... maybe there is an issue with XMP I don't know about.)

One general issue with using mail addresses as identifiers, of course, is privacy. You tag a photo, send it to your brother, he passes it on to his friends, one of them puts it on flickr... and suddenly the mail address of the person on the image is publicly available as a tag on flickr.
This shouldn't happen.

FOAF faced the same challenge. They found a way to securely cipher the mail address so that it's still unique, but will come out as a meaningless string of letters and numbers. Only if you already have the same mail address in your contacts you can compare the two by using the same algorithm... but it's not possible to reverse the process.
So, "person:mail=joe@his.net" would be changed within the image to "person:mail=eea5dd38a317c5051438e70bdfa8f3ccfbffa 88d", and we certainly don't want to see this in any tag/title/description. This is why i think solution b) or c) would be more appropriate.

It's a cool thing, really. I remember there were discussions about solution c) on various mailing lists some years ago; don't know why I never got to see a real-world implementation. I hope it was only people being too lazy, not some real technical problem.