Version: 3.97.1 (using KDE Devel)
Installed from: Compiled sources
".hidden" is a file that contains a list of files and/or directories which doesn't start with a period "." to be treated as hidden by the file manager.
This method is already used by Gtk and GNOME and also Mac OS X and it is a good way to for example to hide system directories in the / filesystem, and also doesn't expose Unix filesystem hierarchy complexity to novice users.
Also with this method you don't force some files or directories to be hidden, it can be customized per distribution and also advanced users can remove or edit this file.
Kubuntu already uses a patch for this in KDE 3.5, you can see some details at:
This can be a very nice feature to be present in KDE4.
This looks like a duplicate of https://bugs.kde.org/show_bug.cgi?id=64740
*** This bug has been marked as a duplicate of bug 64740 ***