netinst
Overview
It is possible to install Ubuntu from the internet by booting from a USB stick or a CDROM. This method of installation is called netinst in Debian and that name is used here as well.
Getting the Image
The images for this are stored in http://archive.ubuntu.com/ubuntu/dists/karmic/main/installer-amd64/current/images/netboot/ (for 64 bit) or http://archive.ubuntu.com/ubuntu/dists/karmic/main/installer-i386/current/images/netboot/ (for 32bit). The CD image is called mini.iso (~12MB) and the image for usb keys is called boot.img.gz (~12MB)
Preparing for Boot
The ISO can be burned to CD with any standard disc burning software, just be sure the ISO is being used as a source image and not just a file on a data disk. To prepare a USB stick, back up any data that you have on it (any data will be overwritten) and zcat boot.img.gz >> /dev/sdX where X is the letter of your USB stick.