If you do a lot of Ubuntu installations, or you like to always carry a copy with you so you can use Linux wherever you are, installing Ubuntu on a USB stick really isn't all that hard.
Simply use Synaptic to install "usb-creator". Download an image of Ubuntu that you want to run from the USB drive, keeping it in an ISO image file. From the System+Administration menu you'll see "Create A USB Startup Disk." Choose the ISO file you downloaded as the source image, and pick your empty USB stick as the destination.
If you chose "Stored in reserved extra space" you can literally run Ubuntu from the stick and any changes you make or files you create will be stored on there as well - in my case my Wireless setup is remembered from boot to boot, and any files I save in my home directory are still there next time.
It's a great way to show off Linux to someone who has never seen it - simply keep it on your keyring and you're always ready to run Linux. It's a great way to carry PC and network diagnostic utilities with you as well.
One caveat - for some reason my Ubuntu 8.10 laptop refused to properly create a boot disk. The created drive would never find the kernel to load. Creating it from my 8.04 desktop worked just fine. I'm unsure where the problem lies. It's even possible that my laptop just won't boot properly from the USB and the drive was actually created just fine.