Why not use the existing category system for tags, and JavaScript for "rollover"? http://commons.wikimedia.org/w/index.php?title=Image:Valeriana_officinalis_2...
Yeah, Magnus' contraption is already usable on commons. It just hasn't had its breakthrough yet. I find the limitation to categories to be a hindrance. I'd prefer arbitrary Wikilinks (or even multiple language links to Wikipedias per tag). This would dramatically increase the number of taggable objects per image and make tagging more attractive IMO.