To install linux from a USB stick:
Make sure the USB drive does not contain any data you want to keep
Download the ISO for the distibution you want to use.
Make sure the USB drive is not mounted:
sudo dd if=ubuntu-11.10-desktop-amd64.iso of=/dev/sdb oflag=direct bs=1M
sudo apt-get install unetbootin
… or download http://unetbootin.sourceforge.net/ and make the binary file
unetbootin-linux-506 executable. (I used Nautilus to do this because
chmod +x didn’t seem to work).
unetbootin (or the file you downloaded).
Select the ISO you want to use.
Type, USB Drive
If you like, UNetbootin will also download a distibution for you.