I've been playing with something similar (havent' we all?). My solution is based on Win32 but I think the same idea will work for *nix. What I wanted was to have a system that in the end, regular applications could access the pictures through there tags. This meant either hacking into the windows file system (which seemed scary and dangerous) or abusing the current system. So I set out to abuse the current system! ;) It turns out that you can do Hard links in windows if you download the right tools. So i have a script monitor a directory, and when a picture is added, it takes all the directories and uses those as tags. For instance if i put a picture in 2005/March/Eric/Mountains it would be tagged with all four of those. Then hard links are distributed out into the other approriate directories. In this case a hard linke would be put in 2005, March, Eric, Mountains, 2005/March, 2005/Eric, 2005/Mountings, March/2005, March/Eric, etc. I'm still working on getting the monitoring right, but right now it looks very promising and exciting. I get the heirarchial access method of starting with broad tags and then as you enter directories you get pictures more and more related to what you want. In the end I want to then tack a web based interface to allow easier tagging and searching of pictures.

I know this isn't quite what you want sense i don't have sub-tags likey are thinking, but maybe there is some way to weld the two ideas together ;). Of the top of my head sub categories could be folders prefixed with something like "." so that the two systems could be merged. Then when storing the categories in a DB you concat the main category with the sub category and store them that way. /me stops thinking out loud.