FYI, just got this panic:
vgonel: not clean, vp 0xcb012d94

trace (copied by hand):
cpu_Debugger
panic
vgonel
vflush
ffs_flushfiles
ffs_unmount
dounmount
vfs_unmountall
vfs_shutdown
cpu_reboot
sys_reboot
syscall_plain

dmesg:
NetBSD 1.5R (HOST) #0: Sun Feb 11 23:43:38 EST 2001
    aidan@host.kublai.com:/usr/local/sup/src/sys/arch/i386/compile/HOST
cpu0: Intel Pentium II/Celeron (Deschutes) (686-class), 348.50 MHz
cpu0: I-cache 16K 32b/line 4-way, D-cache 16K 32b/line 2/4-way
cpu0: L2 cache 512K 32b/line 4-way
cpu0: features 183f9ff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,SEP,MTRR>
cpu0: features 183f9ff<PGE,MCA,CMOV,FGPAT,PSE36,MMX,FXSR>
total memory = 127 MB
avail memory = 115 MB
using 1658 buffers containing 6632 KB of memory
BIOS32 rev. 0 found at 0xfd790
mainbus0 (root)
pci0 at mainbus0 bus 0: configuration mode 1
pci0: i/o space, memory space enabled
Intel 82443BX Host Bridge/Controller (host bridge, revision 0x03) at pci0 dev 0 function 0 not configured
ppb0 at pci0 dev 1 function 0: Intel 82443BX AGP Interface (rev. 0x03)
pci1 at ppb0 bus 1
pci1: i/o space, memory space enabled
pcib0 at pci0 dev 7 function 0
pcib0: Intel 82371AB PCI-to-ISA Bridge (PIIX4) (rev. 0x02)
pciide0 at pci0 dev 7 function 1: Intel 82371AB IDE controller (PIIX4) (rev. 0x01)
pciide0: bus-master DMA support present
pciide0: primary channel wired to compatibility mode
wd0 at pciide0 channel 0 drive 0: <WDC AC313500D>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 12949 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 26520480 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 4 (Ultra/66)
pciide0: primary channel interrupting at irq 14
wd0(pciide0:0:0): using PIO mode 4, Ultra-DMA mode 2 (Ultra/33) (using DMA data transfers)
pciide0: secondary channel wired to compatibility mode
atapibus0 at pciide0 channel 1
cd0 at atapibus0 drive 0: <TOSHIBA CD-ROM XM-6502B, , 1914> type 5 cdrom removable
cd0: 32-bit data port
cd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 2 (Ultra/33)
pciide0: secondary channel interrupting at irq 15
cd0(pciide0:1:0): using PIO mode 4, Ultra-DMA mode 2 (Ultra/33) (using DMA data transfers)
uhci0 at pci0 dev 7 function 2: Intel 82371AB USB Host Controller (PIIX4) (rev. 0x01)
uhci0: interrupting at irq 9
usb0 at uhci0: USB revision 1.0
uhub0 at usb0
uhub0: Intel UHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub0: 2 ports with 2 removable, self powered
Intel 82371AB Power Management Controller (PIIX4) (miscellaneous bridge, revision 0x02) at pci0 dev 7 function 3 not configured
vga0 at pci0 dev 13 function 0: S3 Trio32/64 (rev. 0x54)
wsdisplay0 at vga0: console (80x25, vt100 emulation)
eap0 at pci0 dev 14 function 0: Ensoniq AudioPCI 97 ES1373B (rev. 0x06)
eap0: interrupting at irq 10
eap0: TriTech TR28023 codec; no 3D stereo
audio0 at eap0: full duplex, mmap, independent
midi0 at eap0: AudioPCI MIDI UART
rtk0 at pci0 dev 15 function 0: Accton MPX 5030/5038 10/100BaseTX
rtk0: interrupting at irq 5
rtk0: Ethernet address 00:e0:29:35:4d:2d
ukphy0 at rtk0 phy 7: Generic IEEE 802.3u media interface
ukphy0: OUI 0x000000, model 0x0000, rev. 0
ukphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
fxp0 at pci0 dev 16 function 0: Intel InBusiness Ethernet, rev 8
fxp0: interrupting at irq 9
fxp0: Ethernet address 00:d0:b7:70:7e:c4, 10/100 Mb/s
ukphy1 at fxp0 phy 1: Generic IEEE 802.3u media interface
ukphy1: i82555 10/100 media interface (OUI 0x00aa00, model 0x0015), rev. 4
ukphy1: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
isa0 at pcib0
com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
pckbc0 at isa0 port 0x60-0x64
pckbd0 at pckbc0 (kbd slot)
pckbc0: using irq 1 for kbd slot
wskbd0 at pckbd0: console keyboard, using wsdisplay0
lpt0 at isa0 port 0x378-0x37b irq 7
pcppi0 at isa0 port 0x61
midi1 at pcppi0: PC speaker
spkr0 at pcppi0
sysbeep0 at pcppi0
npx0 at isa0 port 0xf0-0xff: using exception 16
fdc0 at isa0 port 0x3f0-0x3f7 irq 6 drq 2
fd0 at fdc0 drive 0: 1.44MB, 80 cyl, 2 head, 18 sec
apm0 at mainbus0: Power Management spec V1.2
biomask fb4d netmask fb6d ttymask fbef
IPsec: Initialized Security Association Processing.
ukbd0 at uhub0 port 2 configuration 1 interface 0
ukbd0: NOVATEK ORTEK USB Keyboard, rev 1.00/1.00, addr 2, iclass 3/1
wskbd1 at ukbd0 mux 1
ums0 at uhub0 port 2 configuration 1 interface 1
ums0: NOVATEK ORTEK USB Keyboard, rev 1.00/1.00, addr 2, iclass 3/1
ums0: 3 buttons and Z dir.
wsmouse0 at ums0 mux 0
boot device: wd0
root on wd0a dumps on wd0b
root file system type: ffs
IP Filter: v3.4.9 initialized.  Default = pass all, Logging = disabled
wsdisplay0: screen 1 added (80x25, vt100 emulation)
wsdisplay0: screen 2 added (80x25, vt100 emulation)
wsdisplay0: screen 3 added (80x25, vt100 emulation)
wsdisplay0: screen 4 added (80x25, vt100 emulation)
wskbd1: connecting to wsdisplay0
wsmux1: connecting to wsdisplay0

It had also filled up with
nd6_lookup: failed to add route for a neighbor(3ffe:80c0:0220::0006), errno=17
and I notice I have a few
Feb 28 03:38:56 host /netbsd: rtk0: watchdog timeout
in my syslog output.

--aidan