- I know that this is not a disklabel problem.  sd@1 has been in here
     for ages.  But FWIW, yes, they're all aligned.
   
   - The message comes from the PROM, not the bootloader.

FWIW, it is the *PROM* that cares about partitions starting on cylinder
boundaries (it doesn't care whether they are aligned at the end).
   
   Is this a symptom, perhaps, of only certain kinds of SCSI disks having
   been "blessed" by Sun?  I can't see that, really; all disks in question
   are SCA -- the other three are 50-pin ribbon SCSI (narrow SCSI-2) in
   a tower, and it doesn't give me any hassle about them.

i've never heard any such thing.  BTW, have you reinstalled bootblocks and
re-labelled the disk, just to make sure?