Earlier sparcs remap SCSI IDs 0 and 3 in OpenBOOT. Linux ignores this. SILO then proceeds to install on the lowest SCSI ID. If you have a drive on 0 and 3, SILO will install on 0, the sparc will try to boot off of ID 3.
Jeff, is this an installer issue (i.e. parameters written to the SILO config file) or a silo issue itself?
Fixing this problem in silo would require knowing all the Sun OBP versions in order to take appropriate corrective measures. It's easier just to fix this problem in the documentation by saying that on older suns's, you should make sure that that unit numbers are the same as the target ID's by setting the appropriate OpenPROM variable before using silo.