Anyone who administers UNIX servers, soon learns the usefulness of screen(1).  In case you haven’t used screen here is a brief description.  screen is a command line utility that allows you to start a command line sesson, start a long running command and detach from the session, leaving the command running.  You can then later reattach to that same session and check out the results.
xpra is the X equivalent of  screen.  Like screen it lets you start an X application in a session, detach from the session, and come back to it later.   It’s still beta, but work a look.
xpra homepage: http://code.google.com/p/partiwm/wiki/xpra
xpra readme: http://code.google.com/p/partiwm/source/browse/README.xpra