Your motherboard must support booting form a USB drive.
My USB drive shows up in the BIOS setup as an ide drive. Not sure if that qualifies as usb-boot support or not. Anyway, it starts to boot off the usb drive, but doesn't get off the ground.
I'm using an ASUS M2NBP-VM CSM. Not sure if the motherboard is the problem, but it seems to be a good board otherwise, ie, it runs monowall 1.3b2 very well off a cf card under decent load.