Domi is a tool for building Xen domains. It's usage is covered in HOWTO Xen and Gentoo hosted on the gentoo wiki site. It says you can use it to install Ubuntu, but no where actaully tells you how. So.. here's how!
# cat ./ubuntu.cfg
DOMI_XEN_CONF="/etc/xen/test"
DOMI_XEN_KERNEL="/boot/vmlinuz-2.6.16.18-xenU"
DEBIAN_MIRROR="ftp://ftp.ubuntu.com/ubuntu/"
DEBIAN_RELEASE="dapper"
DOMI_DISTRO="debian"
DOMI_NAME="test"
DOMI_VDISK="hda"
Yes, I do know the DOMI_DISTRO is set to debian, but ubuntu is based on the debian package manager, so that's correct.
Note that you have to setup the /etc/hostname and /etc/hosts after.