Comment 27 for bug 1066410

Further to post #24, since I'm already using gnome-shell as my primary DE, the workaround (more of an alternative than a workaround) I employed was to switch to gdm.
$ sudo dpkg-reconfigure gdm

I lost the pretty login, but gained a working Lockscreen. Lockscreen was also failing to do anything when selected while logged in with lightdm.