There was a directory in /var/lib/defoma/fontconfig.d called "M":
/var/lib/defoma/fontconfig.d/M containing two font measurement files. When the installer tried to delete /var/lib/defoma/fontconfig.d, it couldn't because the files were there. I don't know what installed them, but when I removed the "M" directory, fontconfig-cong installed.
ProblemType: Package
DistroRelease: Ubuntu 12.04
Package: fontconfig-config 2.8.0-3ubuntu9
ProcVersionSignature: Ubuntu 3.0.0-17.30-generic 3.0.22
Uname: Linux 3.0.0-17-generic i686
ApportVersion: 2.0.1-0ubuntu7
Architecture: i386
Date: Mon Apr 30 21:16:22 2012
DuplicateSignature:
Setting up fontconfig-config (2.8.0-3ubuntu9) ...
rmdir: failed to remove `/var/lib/defoma/fontconfig.d/': Directory not empty
dpkg: error processing fontconfig-config (--configure):
subprocess installed post-installation script returned error exit status 1
ErrorMessage: subprocess installed post-installation script returned error exit status 1
PackageArchitecture: all
SourcePackage: fontconfig
Title: package fontconfig-config 2.8.0-3ubuntu9 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: Upgraded to precise on 2012-04-30 (0 days ago)
I seem to be having a similar problem, except that I have the file id-cache /var/lib/