d-i preseed/late_command string wget finish-script-url -O /target/tmp/finish.sh && in-target chmod +x /tmp/finish.sh && in-target /tmp/finish.sh

# finish.sh
#!/bin/sh
# Install sudo-ldap
env SUDO_FORCE_REMOVE=yes /usr/bin/apt-get -y install sudo-ldap || exit $?
# Puppet handles most of this for me..
# User/bootstrap setup
cat > /etc/nsswitch.conf << EOF
# Template snippet: nsswitch.conf
passwd:		files ldap
group:		files ldap
shadow:		files ldap

hosts:          files dns mdns
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis
EOF