There are a few ways to install Ubuntu without a CD. The following worked for me using an ISO:

1. Create a directory named ubuntu on a spare partition
2. Download the alternate cd ISO via http://releases.ubuntu.com and place in ubuntu directory
3. Download vmlimuz and initrd.gz from http://archive.ubuntu.com/ubuntu/dists/intrepid/main/installer-i386/current/images/hd-media/ to ubuntu dir
4. Add the following to grub:

title installer
root (hd0,2)
kernel /ubuntu/vmlinuz
initrd /ubuntu/initrd.gz

Note: hd0,2 = /dev/sda3, my spare partition

For some reason this only worked for 8.10 intrepid, not 8.04 hardy. I had to use https://help.ubuntu.com/community/Installation/NetbootInstallFromInternet for 8.04.