I used the --mirror argument, eg:
sudo pbuilder create --debootstrapopts --variant=buildd --configfile ~/.pbuilderrc --mirror http://localhost:3142/us.archive.ubuntu.com/ubuntu/ main restricted universe multiverse
verify the caching hits on the proxy:
$ tail -f /var/log/apt-cacher/access.log
Thu Dec 19 19:16:33 2013|26140|::ffff:127.0.0.1|HIT|1272844|us.archive.ubuntu.com_ubuntu_dists_precise_main_binary-amd64_Packages.bz2
Thu Dec 19 19:18:45 2013|26647|::ffff:127.0.0.1|HIT|49563|us.archive.ubuntu.com_ubuntu_dists_precise_Release
Thu Dec 19 19:18:45 2013|26663|::ffff:127.0.0.1|HIT|198|us.archive.ubuntu.com_ubuntu_dists_precise_Release.gpg
Thu Dec 19 19:18:46 2013|26707|::ffff:127.0.0.1|HIT|1272844|us.archive.ubuntu.com_ubuntu_dists_precise_main_binary-amd64_Packages.bz2
--mirror
option that i guess sets the mirror to use. Try setting it to apt-cache-ng server address.http://servername:3142/ubuntu
as your mirror url.