Talking to William (upstream developer), i guess he found - and fixed - the problem. Although gdm socket has changed its place, gnome- screensaver is not supposed to hang because of this.
The history is here: http://bugzilla.gnome.org/show_bug.cgi?id=407524 -- screen does not unlock after locking https://launchpad.net/bugs/76632 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
