Wow my first response! And a need for my correction.
In the post below is a response from a geoTagger with Flickr. Apparently what ever you pass into Flickr in terms of EXIF content you get back? Also there is a hint that Flickr may soon pass along the Flickr Tags as an option but then again the potential compexity of the "tagging" might overwhelm EXIF standards?
EXIF has all sorts of potential but its design is a tad complex. Adobie has a companion file design that might be a "next step" standard? Will think about this a bit more and post some info in a week or so.
Time will tell....