i'm not sure that you can boot off a USB drive. i'm almost positive that you can't. but not certain. I almost recall reading that USB drives were not meant to be booted from. for that matter, firewire drives initally couldn't be booted from, i think this has since been corrected.
there really is no need to boot off a zip, when you can boot off a cd.
i know that i can boot off my scsi zip, but that is another story (on a g3 350, or PM7200).
you should check the apple til library about this.
so you've triend making a boot disk using zip tools?