Poor Man's Union Mount
(commentary on OLPC applications management)
1) there shall be a daemon
2) the daemon sets up inotify watches on /applications
3) when a new directory shows up, it creates symlinks from /usr/bin/app-cmd to /applications/appdir/app-cmd
4) same thing happens for libraries, manpages.
5) UI data files (sounds, icons, glade files) -- some will have to be symlinked or copied, some can stay "private".
6) notifications have to be posted to the desktop environment: .desktop files, MIME magic files, MIME icons, application icoms -- touching a directory may be enough (because some other daemon-like entity will pick up that signal and take it from there).
7) if there's already an application with the same name, we can create /usr/bin/app-cmd-v1.2 and /usr/bin/app-cmd-v2.0 symlinks in addition to moving /usr/bin/app-cmd to point to the newest version of the app.
Think of this sorta like udev for applications...