Multiple parentage for tags would be a very useful feature... it would make tagging much more elegant in many situations. If this needs more explanation than given below, please let me know.

The logic should be relatively simple to modify to incorporate searching for it, since heirarchy of tags would still go only one way.

Example:
I would like to tag photos of bars so that I can find all photos taken inside of bars... I'd like to tag them by bar name as well, but the bar name should be tagged as a subtag of its geographical location (e.g. Places -> Europe -> Switzerland -> Geneva -> Old Town -> Flanagan's). Currently, the only way to get around this is to create a new generic tag "Bars", to add to all of them, when in reality, all Bar tags should simply be a subtag. Multiple inheritance is necessary for this.