Although it would be odd to install lightdm without X, I think lightdm should install successfully in a pbuilder environment. In this case (log below) it either needs to make X a dependency or simply create /etc/X11 if it is not present first. Personally I vote for option 2.
This was in an oneiric i386 pbuilder environment.
root@caprica:/# apt-get install lightdm
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
dbus libxau6 libxcb1 libxdmcp6
Suggested packages:
dbus-x11
Recommended packages:
xserver-xorg unity-greeter lightdm-greeter
The following NEW packages will be installed:
dbus libxau6 libxcb1 libxdmcp6 lightdm
0 upgraded, 5 newly installed, 0 to remove and 0 not upgraded.
Need to get 710 kB of archives.
After this operation, 2384 kB of additional disk space will be used.
Do you want to continue [Y/n]? Y
Get:1 http://archive.ubuntu.com/ubuntu/ oneiric/main libxau6 i386 1:1.0.6-3 [8258 B]
Get:2 http://archive.ubuntu.com/ubuntu/ oneiric/main libxdmcp6 i386 1:1.1.0-3 [12.9 kB]
Get:3 http://archive.ubuntu.com/ubuntu/ oneiric/main libxcb1 i386 1.7-3 [44.3 kB]
Get:4 http://archive.ubuntu.com/ubuntu/ oneiric/main dbus i386 1.4.14-1ubuntu1 [549 kB]
Get:5 http://archive.ubuntu.com/ubuntu/ oneiric/main lightdm i386 1.0.1-0ubuntu6 [95.9 kB]
Fetched 710 kB in 2s (253 kB/s)
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously deselected package libxau6.
(Reading database ... 14939 files and directories currently installed.)
Unpacking libxau6 (from .../libxau6_1%3a1.0.6-3_i386.deb) ...
Selecting previously deselected package libxdmcp6.
Unpacking libxdmcp6 (from .../libxdmcp6_1%3a1.1.0-3_i386.deb) ...
Selecting previously deselected package libxcb1.
Unpacking libxcb1 (from .../libxcb1_1.7-3_i386.deb) ...
Selecting previously deselected package dbus.
Unpacking dbus (from .../dbus_1.4.14-1ubuntu1_i386.deb) ...
Selecting previously deselected package lightdm.
Unpacking lightdm (from .../lightdm_1.0.1-0ubuntu6_i386.deb) ...
Setting up libxau6 (1:1.0.6-3) ...
Setting up libxdmcp6 (1:1.1.0-3) ...
Setting up libxcb1 (1.7-3) ...
Setting up dbus (1.4.14-1ubuntu1) ...
invoke-rc.d: policy-rc.d denied execution of start.
Setting up lightdm (1.0.1-0ubuntu6) ...
Adding group `lightdm' (GID 103) ...
Done.
Adding system user `lightdm' (UID 102) ...
Adding new user `lightdm' (UID 102) with group `lightdm' ...
Creating home directory `/var/lib/lightdm' ...
usermod: no changes
usermod: no changes
usermod: no changes
Adding group `nopasswdlogin' (GID 104) ...
Done.
/var/lib/dpkg/info/lightdm.postinst: 66: cannot create /etc/X11/default-display-manager: Directory nonexistent
dpkg: error processing lightdm (--configure):
subprocess installed post-installation script returned error exit status 2
Processing triggers for libc-bin ...
ldconfig deferred processing now taking place
Errors were encountered while processing:
lightdm
E: Sub-process /usr/bin/dpkg returned an error code (1)
# simple fix:
root@caprica:/#
root@caprica:/# mkdir -p /etc/X11
root@caprica:/# apt-get install lightdm
Reading package lists... Done
Building dependency tree
Reading state information... Done
lightdm is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue [Y/n]? y
Setting up lightdm (1.0.1-0ubuntu6) ...
root@caprica:/#
Will be part of the next release