We dont use KDM, and as such ubiquity gets axed along with remastersys. Edit your /usr/bin/remastersys file to comment out the lines above and it'll leave ubiquity alone in the default configuration. I've had mine set up like this for a while and have been making progressive ISO sets of my decently customized OS every time i test my changes (helped today when i decided to rollback due to a heap smashing error with nmap which i use daily).
if [ "`cat /etc/X11/default-display-manager`" = "/usr/bin/kdm" ]; then
apt-get -y -q install ubiquity-frontend-kde
apt-get -y -q remove ubiquity-frontend-gtk
apt-get -y -q install ubiquity-frontend-gtk
apt-get -y -q remove ubiquity-frontend-kde
apt-get -y -q install metacity