Hm, I just realized something. You said this works for directories too. Does that mean that dpkg has changed for this so it (if statoverride is not used) always changes the permissions of directories when upgading packages that contain them.
Note of course, that it currently does not. I'm not sure if doing so would be a good idea; it'd be easier on maintainers, but would violate least suprise for admins. -- see shy jo
