I don't know if this applies to single processor. but this kernel
was build using up to date anoncvs, and it paniced. A day ago
the same kernel just hung with no info as it was trying to initially
mount filesystems, so there is some type of change or instability.

anyways, here is what I managed to get from a trace

panic+174
trap+8ec
kernel MCHK trap by route_output+c
raw_usrreq+1e8
pmap_pvo_enter+244
interhand+7c
route_usrreq+194
sosend+728
soo_write+2c
dofilewrite+d0
sys_write+94
syscall_plain+138
s_sctrap+138

I couldn't get sync to flush the disks

I have also been able to hang my sort of stable MP kernel from about a 
week ago with
a looping program while doing other things. The console hangs and I 
can't do anything
but reset the machine to get the console back. I think that one 
processor was still working
away at a compile, but the keyboard hangs, and the debugger is not 
responding.
Perhaps somewhere there is still a non-smp-aware routine in either the 
debugger or
console stuff which can't tell who to talk to?

system is a powermac dual 1GHz G4 (older style, whatever its name is)

Hope this helps debugging. Let me know if other tests are in order.