If your NetBSD disk is the first drive on the internal bus(sd@0:0) AND
sysinst installed the boot blocks correctly then boot scsi-int/sd@0:0
should boot your machine into NetBSD. If it doesn't then reboot (boot
fd:0 from OF) from your boot floppy and 
 mount your root (/) filesystem on /mnt
 cd to /usr/mdec (of the install sys)
 run ./installboot /mnt/boot /usr/mdec/bootxx /dev/((root
device/partition - sd0a in my case))

This has let me boot 20000502 (various from ftp), 1.4.2, 1.4.3a, and 1.5
- but with each one I get the infamous:

mesh:timeout state=3
mesh: resetting dma
and a big nasty hard freeze until powerdown....

Maybe someone more knowledgeable of NetBSD could tell me if the second
processor has any effect? (I have a s900MP)

(note: if your drives have not spun up yet, you might have to change
your OF boot-command to:
 begin ['] boot catch 1000ms cr again 
(this sets a delay for the disk))

Tom