I believe that your options for boot devices is entirely limited by your motherboard's BIOS. I've seen motherboards that limited boot devices to just IDE disks and floppy disks while other motherboards include options for booting from SATA, SCSI, and network devices. Some motherboards have an option something like "Look for other boot devices" to catch any options that were not explicitly listed. Note that motherboards that are made for the computer hobbyist market are more likely to have more options. Motherboards made for particular brands may have fewer options.
In the case of USB devices you will want to look for an option to boot from SCSI devices.
If all else fails you might want to make an initrd style Linux boot floppy disk that would have support for USB and SCSI. That could possibly load a kernel off of a USB disk. I am not very knowledgeable about initrd yet but I do know that it could be used as I said. Of course the real kernel on the USB disk would also have to have USB and SCSI disk support built into it.
So look on your motherboard's CMOS BIOS settings for boot devices. Enable booting from SCSI devices and, if there is such a setting, enable looking for additional boot devices that are not specifically listed in the BIOS boot device list.
I would also expect better results if you were trying to boot from a USB port that is built onto the motherboard rather than booting from a USB add on card.