There are two use cases for moving windows between displays (WBUS) which we are interested in:
 
1) Multiple users, single app (MUSA).

2) Single user, multiple machines (SUMM)


For the multiple user scenario, the idea is that users are sharing a window, as in:
User A and B talk about app X, A knows X, B doesn't, so A runs X on his machine, and
invites B to share the window. This may be done by selecting a "Share this window with..." 
menu item from the window manager menu. B would see a notification on his display, asking
him if he wants to share the window. While the app is shared, B should be able to
watch As interaction with the window, including popups, dialogs etc. It would also be nice
if A could allow B to interact with the window as well. This use-case sounds a lot like
per-window-vnc.

For the single user scenario, think about laptop/desktop or laptop/projector cases.
The idea is that each app has a "home display" (the one where it was initially started, 
should be the local display most of the time). If you are on a remote display you should 
be able (to bring up a list of available remote displays), select one and (after auth)
pick a window from a list of windows on that display to be pulled over. Pulling the window
over should leave a "shadow" (maybe a grayed out screenshot of the image before moving)
with a "bring back" button. The pulled over window should operate as normal, the main
difference being that it should have a "give back" button instead of a "close" button in 
the window frame. A more star-trek way to select a window to be pulled would be to display
a miniature view of the selected remote display, and allow to drag a window from the remote
display onto the background.