Comment 3 for bug 819954

For what its worth the modified conffiles function will know if most of the files in /etc/lightdm/ modified except for /etc/lightdm/lightdm-gtk-greeter.conf which is not a lightdm conffile.

7:49:46 - flash:[~/photos/new/canon-xti/07-july] dpkg-query -W --showformat='${Conffiles}' lightdm
 /etc/lightdm/users.conf 8d76f0b67a0b6bf02464c2964deccdc9
 /etc/pam.d/lightdm 02d4bd40eb6d8c03d4063da806aaf42c
 /etc/pam.d/lightdm-autologin 3d4f9d79e7302b4eff2b7a00e5e4693c
 /etc/init/lightdm.conf 7a30efa5ebebadd80fe78d21e8266588
 /etc/dbus-1/system.d/org.freedesktop.DisplayManager.conf e2699292bde676d83e2985a61b7f80b5
 /etc/lightdm/lightdm.conf 2c462cff90dda9d900add7187930ba00 obsolete
 /etc/lightdm/Xsession a7475cc7476a26be823c094c00f618c0 obsolete

So if that file is important it'd be worthwhile to include it in the package hook or get it added as a conffile.