I'm running Lion with the latest updates, and the X11 clipboard has been working for some time now - Select some text in a terminal, press ⌘-C, change to for example Firefox, and press ⌘-V. This simply doesn't work anymore. I've tried enabling and all of the X11 Pasteboard preferences, but no luck. It will take some time to get all the terminals up to speed if I have to restart X11, so "restart/reboot/reinstall" is not an option.
Copying between terminals and between GUI apps works, and also from GUI applications to terminals. Only terminals → GUI doesn't work.