(This is just a forward of an email I sent to someone, this case was dealing
with a SS5, and am not sure if it can be any help on another sun system.
But hey figured it could be at least something useful to know.)

Hey-

I had the same trouble with mounting the / partition as read only on boot.
What I found the problem to be was that the prom was booting up from the c
partition instead of the a partition.  Since c was the whole disk, and a was
at the beginning of the disk it would boot up fine but with no rw access.
What you need to do is go into the prom before the system boots and make a
few adjustments.  I did these 3 steps and it since has worked for me.  You
need to get yourself to the 'ok' prompt, break a if memory serves me is the
correct way.

ok printenv
ok setenv nvramrc devalias unix /iommu/sbus/espdma/esp/sd@3,0:a
ok setenv boot-device unix

-Jesse