Bit of an oddish one, if I've opened an X window over SSH in a screen session will it reopen when screen is reattached? I've been able to detach and the process stays running fine, with the X window continuing to update with me logged onto the server but I'm just curious if I'll be able to detach and logoff, then reattach and bring the window back.
If I understand you correctly, you logged into a remote system with SSH with X forwarding turned on, and then started up an X application and sent it to the background. If you log off completely, the SSH tunnel that provides the X forwarding feature will also close, and the X app will be terminated. However, most likely what will happen when you try to log off is that the SSH session will remain open until the X application exits. The screen session is inconsequential at this point.
Interestingly enough, there appears to be some work on being able to save and restore X sessions like screen does with terminal sessions.