There's a quick and easy method of installing Ubuntu (or any derivative) from a pendrive: isohybrid.
It's a tool which is part of the syslinux package.
just install it, then type
Code:
isohybrid FILENAME.ISO
where FILENAME.ISO is your iso file. The md5 checksum will change, of course.
Then, you can just write your modified iso file to a pendrive:
Code:
sudo su
umount /dev/sdx
where sdX is your pendrive device name. Pay attention!
Code:
dd if=FILENAME.ISO of=/dev/sdX
sync
I wonder why the standard ISOs aren't already patched with this. Or maybe they are and I didn't know it?

Hope this helps