This for some reason intrigues me. A lockable box that can only be removed via the inside of the box. Of course, these things would not stop a concentrated effort (a dremel would make short work of it), but does afford (relative) security for bike accessories that one normally have to lug around off the bike. Some examples:

-Planet Bike Escape Pod
-Cargo Cache. Found this by post here on BF
-Swagman Whee Pod
-Sunlite's offering

Major concerns that I have are that they might block any seat post light/reflector, and hamper access to a wedge pack.