If you want a step-by-step proceedure to create a bootable USB, try:
0) Toss unetbootin.
1) Boot from the live DVD.
2) Make a directory for your USB mount point => mkdir /mnt/sda1
3) Make a directory for your CDROM mount point => mkdir /mnt/cdrom
4) Stick in your USB flash drive (I use a 2 GB Kingston for my USB).
5) Mount your USB => mount /dev/sda1 /mnt/sda1
6) Partition your USB => fdisk /dev/sda1 (just use one large partition)
7) Format your USB => mkfs.ext3 /dev/sda1
8) Mount your DVD => mount /dev/hdc /mnt/cdrom (it may already be mounted in /media).
9) Copy all the contents from the DVD to the USB drive => cp -vr /mnt/cdrom /mnt/sda1
10) Navigate to /mnt/sda1
11) Run GRUB => grub-install --recheck --no-floppy --root-directory=/mnt/sda1 /dev/sda1
12) Navigate to /mnt/sda1/boot/grub and run => cat menu.lst
13) Check the menu to see if has the correct pointers (it ususally will).
14) Run nano on the menu to correct the screens => nano menu.lst
15) Add vga=0x317 at the end of the kernel line for the 1024x768 listing.
16) Add vga=0x314 at the end of the kernel line for the 800x600 listing.
The particulars listed may not be idiot proof - I'm assuming that you have some slight knowledge of linux and will understand the commands and what extra to do. This is what I do, and I have two USB disks that I update like this all the time.