What X does not AFAIK have and has never had is a way to
transparently move a client from one server to another.  Adding
some sort of hack to a window manager so it will start forwarding
one particular client to another server seems doable within current
infrastructure.  But the original server that the client thinks
it is displaying on must remain in the picture as a middleman.  VNC,
used to export an X server, is a heavy-handed example of this concept
since it forwards the whole server instead of just one client at
a time.

A more intruiging infrastructure hack that might allow moving an X client
from one server to another WITHOUT MODIFYING THE X CLIENT IN ANY WAY
is the proposed TCP extension for migrating one end of a connection.

"etcp" it is called, and the demonstration page including BSD patches,
which worked in 2001, appears to be down right now.

http://lists.insecure.org/linux-kernel/2001/Jul/4301.html