There are different methods to create an installation boot disk under Linux. Bootable floppy disks are generally used as a last resort to boot the installer on hardware that cannot boot from CD or by other means such as a BIOS limitation.
Here is a very short (but in my opinion very useful) how-to for creating an USB boot device, which enables you to boot Debian Linux from your memory stick or USB pen.
We are assuming that your USB device is indicated by /dev/sda, and that you are interested in Debian stable (other distributions are untested, but should work).
If you are a new to USB pen under Linux, read this mini-how-to which explains how to use USB pen/flash stick.