FWIW, I've narrowed down the possibilities. With pciide enabled, I see the
following sort of errors. With a kernel built from the same sources but using
wdc instead of pciide, I don't get the errors. (However, the box has only been
up a bit shy of two hours with the non-pciide kernel, so it's conceivable that
I'm just bucking the odds a bit and that the error is still going to happen.
The more time passes, the less likely this seems, however.)

Jan 29 02:05:00 acheron /netbsd: Data modified on freelist: word 0 of object 0xf0609970 size 4 previous type UVM amap (0xfeadbeef != 0xdeadbeef)
Jan 29 02:10:00 acheron /netbsd: Data modified on freelist: word 0 of object 0xf0609610 size 8 previous type UVM amap (0xfeadbeef != 0xdeadbeef)
Jan 29 08:45:00 acheron /netbsd: Data modified on freelist: word 0 of object 0xf06095b0 size 4 previous type UVM amap (0xfeadbeef != 0xdeadbeef)
Jan 29 10:53:54 acheron /netbsd: Data modified on freelist: word 0 of object 0xf06098f0 size 16 previous type UVM amap (0xfeadbeef != 0xdeadbeef)

If I can provide any useful information or act as a guinea pig for test code
to track down the error, I'll be happy to do so.

Here's my dmesg:

NetBSD 1.4Q (MLB) #5: Fri Jan 28 14:22:23 EST 2000
    root@acheron.middleboro.ma.us:/usr/src/sys/arch/i386/compile/MLB
cpu0: family 5 model 8 step c
cpu0: AMD K6-2 (586-class)
total memory = 127 MB
avail memory = 116 MB
using 1659 buffers containing 6636 KB of memory
BIOS32 rev. 0 found at 0xfb380
mainbus0 (root)
pnpbios0 at mainbus0: code f0000, data f0000, entry c088
pnpbios0: 15 nodes, max len 98
PNP0000 (io 20-21 a0-a1, irq 2) at pnpbios0 index 0 ignored
PNP0200 (io 0-f 81-83 87 89-8b 8f-91 c0-df, dma 4) at pnpbios0 index 1 ignored
PNP0100 (io 40-43, irq 0) at pnpbios0 index 2 ignored
PNP0B00 (io 70-71, irq 8) at pnpbios0 index 3 ignored
PNP0303 (io 60 64, irq 1) at pnpbios0 index 4 ignored
PNP0800 (io 61) at pnpbios0 index 5 ignored
PNP0C04 (io f0-ff, irq 13) at pnpbios0 index 6 ignored
PNP0C01 (mem 0-9ffff fffe0000-ffffffff 100000-7ffffff) at pnpbios0 index 7 ignor
ed
PNP0C02 (mem f0000-f3fff f4000-f7fff f8000-fbfff fc000-fffff) at pnpbios0 index 
8 ignored
PNP0A03 (io 4d0-4d1 cf8-cff 480-48f 4000-407f 4080-40ff 5000-501f) at pnpbios0 i
ndex 9 ignored
PNP0C02 (io 208-20f) at pnpbios0 index 11 ignored
com0 at pnpbios0 index 12 (PNP0501)
com0: io 3f8-3ff, irq 4
com0: ns16550a, working fifo
PNP0700 (io 3f2-3f5, irq 6, dma 2) at pnpbios0 index 13 ignored
lpt0 at pnpbios0 index 15 (PNP0401)
lpt0: io 378-37f 778-77a, irq 7, dma 3
com1 at pnpbios0 index 16 (PNP0501)
com1: io 2f8-2ff, irq 3
com1: ns16550a, working fifo
pci0 at mainbus0 bus 0: configuration mode 1
pci0: i/o space, memory space enabled
pchb0 at pci0 dev 0 function 0
pchb0: VIA Technologies VT82C598 (Apollo MVP3) Host-PCI (rev. 0x04)
ppb0 at pci0 dev 1 function 0: VIA Technologies VT82C598 (Apollo MVP3) PCI-AGP (
rev. 0x00)
pci1 at ppb0 bus 1
pci1: i/o space, memory space enabled
pcib0 at pci0 dev 7 function 0
pcib0: VIA Technologies VT82C596A (Apollo Pro) PCI-ISA Bridge (rev. 0x12)
VIA Technologies VT82C586A IDE Controller (IDE mass storage, interface 0x8a, rev
ision 0x06) at pci0 dev 7 function 1 not configured
uhci0 at pci0 dev 7 function 2: VIA Technologies VT83C572 USB Controller (rev. 0
x08)
uhci0: interrupting at irq 11
usb0 at uhci0: USB revision 1.0
uhub0 at usb0
uhub0: VIA Technologie UHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub0: 2 ports with 2 removable, self powered
pchb1 at pci0 dev 7 function 3
pchb1: VIA Technologies product 0x3050 (rev. 0x20)
vga0 at pci0 dev 17 function 0: Nvidia Corporation Riva TNT (rev. 0x04)
wsdisplay0 at vga0: console (80x25, sun emulation)
ne0 at pci0 dev 19 function 0: RealTek 8029 Ethernet
ne0: 10base2, 10baseT, 10baseT-FDX, auto, default auto
ne0: Ethernet address 00:40:05:6a:5e:fe
ne0: interrupting at irq 9
isa0 at pcib0
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
pmsprobe: reset error 5
wdc0 at isa0 port 0x1f0-0x1f7 irq 14
wd0 at wdc0 channel 0 drive 0: <Maxtor 91731U4>
wd0: drive supports 16-sector pio transfers, lba addressing
wd0: 16603 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 34004880 sectors
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 4
wd1 at wdc0 channel 0 drive 1: <Maxtor 90680D4>
wd1: drive supports 16-sector pio transfers, lba addressing
wd1: 6485 MB, 13176 cyl, 16 head, 63 sec, 512 bytes/sect x 13281408 sectors
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 2
wdc1 at isa0 port 0x170-0x177 irq 15
atapibus0 at wdc1 channel 0
cd0 at atapibus0 drive 0: <Memorex CRW-1622, AH1234567892123  L2, D4.0> type 5 c
drom removable
cd0: drive supports PIO mode 3, DMA mode 1
sb0 at isa0 port 0x220-0x237 irq 7 drq 1 drq2 5: dsp v4.13
audio0 at sb0: full duplex, mmap, independent
midi0 at sb0: SB MIDI UART
opl0 at sb0: model OPL3
midi1 at opl0: SB Yamaha OPL3
pcppi0 at isa0 port 0x61
spkr0 at pcppi0
sysbeep0 at pcppi0
isapnp0 at isa0 port 0x279: ISA Plug 'n Play device support
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
isapnp0: read port 0x203
ep0 at isapnp0 port 0x210/16 irq 5
ep0: 3Com 3C509B EtherLink III 
ep0: address 00:a0:24:67:d0:25, 8KB byte-wide FIFO, 5:3 Rx:Tx split
ep0: 10baseT, 10base5, 10base2 (default 10base2)
biomask fd45 netmask ff65 ttymask ffe7
boot device: wd0
root on wd0a dumps on wd0b
mountroot: trying msdos...
mountroot: trying cd9660...
mountroot: trying nfs...
mountroot: trying ffs...
root file system type: ffs
init: copying out path `/sbin/init' 11
ums0 at uhub0 port 2 configuration 1 interface 0
ums0: Logitech USB-PS/2 Mouse, rev 1.00/1.20, addr 2, iclass 3/1
ums0: 3 buttons
wsmouse0 at ums0 mux 0
IP Filter: initialized.  Default = pass all, Logging = enabled
IP Filter: v3.3.6
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, sun emulation)
wsdisplay0: screen 5 added (80x25, vt100 emulation)

-- 
   Mason Loring Bliss   mason@acheron.middleboro.ma.us              E w i g e
awake ? sleep : dream;  http://acheron.ne.mediaone.net  B l u m e n k r a f t