My advice is to keep it simple, don't place to much information about the image in the filename.
Yes, You can trust on the metadata to organize your collection images.
When could append your initials at the beginning of the photo's made by yourself. For other images use the initals of the photographer.Metadata:
The author an copyright fields contains information about the photographer who made this particulair photo, it does not say anything about the subject...
Expression Media does have a field 'People' in where you can put names of the persons on the photograph but i believe it is not a standard field.
In the extended IPTC fields ( http://www.iptc.org/goto?help-cs5panels
) you can put information about the subject but they can not be used for organising your files in Expression Media. Maybe in the future..
In the meentime you can organise your images with catalog sets, like the most of us do.