I have similar issues but none with the 12.04 build.

Im trying this on a dell d610 laptop with a bad hard drive and was told i could just directly boot from the usb drive.

Im using an 8g usb drive and i have tried using the uui tool and get this message. SYSLINUX 4.06 EDD 4.06-pre1 Copyright (c) 1994-2011 H.Peter Anvinet al ERROR: No configuration file found No DEFAULT or UI configuration directive found!

Then i tried using unetbootin and got the bootup screen with only the default option and it would say auto boot with a 10 sec countdown and start over again.

I do have the boot sequence for usb first.