Ubuntu on Bootable USB
MadTux have released a bootable USB Pen Drive:
"1.0GB USB Pen Drive with the entire contents of the Ubuntu 5.10 Live Release and added special code to make it bootable. Just plug it into the USB port of your computer (BIOS capable of booting USB flash is required*) and boot into Ubuntu Linux"
Further information on their site
EDIT: Link updated to point to the category on their site with various linux distros including Ubuntu on bootable USB
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I prefer the tutorials at http://www.pendrivelinux.com myself. They have tons of Live Linux USB tutorials there. :)