On my computer, I had to explicitly set some setting in the BIOS
settings to allow booting from SCSI.  I think some BIOSes don't
support this option, it might be worth checking your motherboard
manual, your SCSI card's manual might have more information as
well.

I boot from my SCSI drive on a Cyrix P200+ w/Intel VX based
motherboard every day. Haven't tried booting from Zip or Jaz yet, I'd
guess it'd just work.

-Zach