A year ago we explained how to use Kubuntu 8.10 from a USB device. Now, a year later, I was in the same situation, but with Kubuntu 9.10 instead. And since then we've made some steps.
Ubuntu itself brings an application to make a USB device boots Ubuntu. In the panel there is the application "K-> Applications -> System -> USB Startup Disk Creator" (o also with /usr/bin/usb-creator-kde command), which is very simple