So, I've installed -current (as of yesterday) and able to net boot my
ss20 with it.  If I use the GENERIC_SCSI3 all looks fine.
If I boot TOO.MP (inlcudes LOCKDEBUG etc) I get core files from:

-rw-------  1 root  wheel  397312 Jan 20 15:20 amd.core
-rw-------  1 root  wheel  593920 Jan 20 15:20 mount_nfs.core

I also see the following at times:

Jan 20 23:34:34 too syslogd: Exiting on signal 15
bogus interrupt ipl 0x6 pc=0xf014eb48 npc=0xf014ea8c psr=1e4004c3<S,PS>
xcall(cpu1,0xf00078dc): couldn't ping cpus:
Jan 20 23:34:34 too syslogd: Exiting on signal 15

But in general it works.  I picked up pk's fix for cpu.c:

     $NetBSD: cpu.c,v 1.168 2003/01/20 00:55:52 pk Exp $

on Jan 19 17:04, but the rest of the sources 
but the rest of the tree dates from Jan 18 16:01.

Interestingly I couldn't boot GENERIC.MP - too big.

net_open: file name: /tftpboot/sparc/boot.net
Using BOOTP protocol: ip address: 192.168.250.2, netmask: 255.255.255.192, gateway: 192.168.250.3
root addr=192.168.250.10 path=/export/diskless/too/root
kernel too large: 3674520 (maximum kernel size is 3670016)
device[/iommu@f,e0000000/sbus@f,e0001000/ledma@f,400010/le@f,c00000] ("halt" to halt): 

I'll cvs update again and see how we go...

--sjg