"Detach" and "Reattach" xterms across X sessions?

"You can use screen within an xterm. This is a program that allows you to attach and detach terminal sessions. So you would start an xterm, start screen within the xterm, and use the xterm as normal. To move the session, you would detach from the session within screen and exit the xterm. Start up another xterm somewhere else, and reattach to the existing screen session.

If you want to do it with any X program, you can use NX. It works like screen, in that you are able to detach/attach to a X server. It also understands the X11 protocol, so it does a good job at speeding up X over slow networks."

No. Out of the box X does not have this facility. If you want to do this the best way is to use XVNC which is an x server that renders to a VNC virtual frame buffer. Then you can connect to your X session using a VNC client.

"Have you looked into xmove?

en.wikipedia.org/wiki/Xmove"

If you only care about the command-line session inside the xterm, use screen