I decided to give sparc64/1.5.1 a try on one of my Ultra2 clone machines and
after I boot miniroot it just drops into a kernel panic.
this machine is a Tatung COMPstation Ultra2 2 x UltraSPARC-II 296MHz with
2048 megs of ram, using OpenBoot 3.11.
it does seem to start booting the kernel as it displays this:

Rebooting with command: boot disk:b netbsd -s
Boot device: /sbus/SUNW,fas@e, 8800000/sd@0, 0:b  File and args: netbsd -s
NetBSD IEE 1275 Bootblock
..>> NetBSD/sparc64 OpenFirmware Boot, Revision
>> (martin@setting-sun.duskware.de, Thu Jun 28 20:24:14 CEST 2001)
loadfile: reading header
elf64_exec: Booting /sbus@1f, 0/SUNW, fas@e, 8800000/sd@0, 0:b/netbsd
330119@0x1000000+221520@0x1400000+433440@0x1436150
symbols @ 0xff08300 74+244944+126870 start=0x1000000
chain: calling OF_chain(80000, ee30, 1000000, fffb1a80, 18)
[ preserving 372664 bytes of netbsd ELF sybol table ]
consinit()
setting up stdin
stdin instance = ffe5e78
setting up stdout
stout instance = ffe5e78
stdout package = f0075380
console is keyboard/display
Copyright (c) ....
	The NetBSD Foundation ....
Copyright (c) ....
	The Regents ....

panic: vm_page_physget: called _after_ bootstrap
kdb breakpoint at 0x1277820
Stopped in  at  cpu_Debugger+0x4:       nop

hope this helps.

Jason Mealins