This is half-bug and half-feature, so let's start with the problem that prompted this topic:

Problem:
If I link to a file in a Folder A, then add a new folder that comes before Folder A alphabetically, the "directory ID" (did as FB calls it) increments to accommodate this new directory.

Solution:
Although it would be hard to keep a persistent state on the did's, it's necessary in one form or another. One way would be to use .htaccess RewriteRules to redirect, say, /index.php/foldername/photoname to the appropriate file.

Thoughts? Any way to do this easily that I'm not thinking of?

Well yes, but then you don't get the nice site interface + the auto-resizing etc. For me that's half the goodness of FB.

What he said ^. I'd like nice-urls too, the ?fpp=10&did=2 blah is horrible.