FWIW, unetbootin did not work for me when I tried a few months ago with Lenny Stable; the boot files for unetbootin had version issues with the Lenny Stable release iso's I downloaded. The Debian Installation Manual instructions, however, worked perfectly (I had posted an email thread not too long ago confirming dvd iso's also work from a bootable usb device using this same method, with a separate FAT partition for booting and an ext3 partition for storing the dvd .iso file):
http://www.debian.org/releases/stable/i386/ch04s03.html.en#usb-copy-flexible
Mark