Well, twospoons, I start from the assumption that categories we use to classify our files (photos, music, videos) have place in PB as thoughts (not as PB categories).
If I like to use PB as personal database (too), I can insert a lot of files as attachments into one specific thought, created as child of all my categories. So, for instance, I'ld create thoughts: '2008,2009,2010'... as time categories, then 'work, home, travels'... as common-place categories, then 'barcelona, sardinia, rome'... as specific places, 'friends, relatives, workmates, wife'... as common people, and so on. I also mark (with some flags to implement on PB) this thoughts for tagging (that is as category source for future linked files). Now after my come-back from holidays in sardinia with my wife in 2008, or from my work in barcelona with my colleagues in 2009, etc., I create specific thoughts for this two events and put all my photos inside them. In facts, after connecting these thoughts with all above general thoughts (each with proper ones), I have already (inside PB) categorized files, so why not to leave PB write these info inside my photos instead of doing it manually and a lot of times? There is a standard for tagging, so PB could write thought names (those ones linked to the event with photos attached and marked for tagging) inside file tags. I hope to have explained better my idea...
Is this your house?
I'm Wiston Wolf, I solve problems.