I can verify that I had the same problem with creating a live USB with the AMD 64 alternate Ubuntu 12.04 and 12.04.1 images
I didn't have a working linux box to use to make the live USB with 'dd' but I was able to successfully use OSX to achieve the same thing.
I had trouble finding a good post on how to do this (I found several conflicting instuctions), so here's what I did:
insert usb drive
check what the DRIVE address for the USB is, mine was /dev/disk2
and you're good to go.
diskutil unmountDisk /dev/disk2
sudo dd if=/path/to/ubuntu-12.04.1-alternate-amd64.iso of=/dev/disk2 bs=1m
Thank you very much znorris, I struggled with this on and off for a couple of weeks and made no progress until I found this thread.