The box: Fujitsu-Siemens Primergy 470 (mainboard D1031): Intel 440GX
  chipset and an LSI sym53c895 scsi HBA, which has a Pioneer DVD-ROM
  (DVD-304) at 0,5,0 and an IBM DDYS-T18350N-S96H hard disk at 0,2,0.
  The Symbios has parity activated and the parity jumper on the disk is
  enabled.
  The NetBSD cdrom (3.1 or 4.0RC2) complains:

  sd0(siop0:0:2:0): parity error
  siop0: scsi bus reset

Several thoughts:

  Your scsi termination might be wrong, or it might be that NetBSD is
  not setting it up right.

  The cdrom might not implement parity, or it might be configured off,
  and the NetBSD driver is somehow doing the wrong thing with two devices.n

Are you sure the other systems are actually doing parity?  If you
don't have a scsi bus analyzer, I'm not sure how you could be.

What happens if you disable parity in the controller or the disk, or
both?

Can you access the cdrom after this parity error?

  so the 0,2,0 disk isn't accessible, and the scsictl utility of a NetBSD
  installed on an ata disk in the same box is of no use.

meaning if you try to detach and then scan exactly what hapens?

I wonder why esiop(4) isn't being used for this chip.