Having read some information on the web and the Sun documentation, I'm
still not sure what's the correct way to set up my X4100 servers to get
the serial console via SSH login to the SP.

I have at the moment set the BIOS to disable redirect (i.e. stop
redirecting before the boot loader starts - if the BIOS documentation is
correct), set the GRUB to serial console output in /boot/grub/menu.lst
and set Solaris via "eeprom console=ttya" to use the serial console.

For the moment everything seems to work, but I had to reset the SP at
least once to get the console working again after I disconnected.