OK, so this is most odd.

I have two new systems. Both have the same mobo; the difference is that one 
system has dual-core CPUs (Opteron 270) and the other doesn't (Opteron 246). 
I've been trying to boot the 270-based system.

For grins, I unpacked the 246-based system and tried Greg's ISO:



 >> NetBSD/amd64 BIOS Boot, Revision 3.3
 >> (oster@quadrapus, Fri Aug  4 13:58:01 CST 2006)
 >> Memory: 632/65535 k
 > boot
booting fd0a:netbsd
3688928+5270360+224176=0x9c2bc8
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006
     The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
     The Regents of the University of California.  All rights reserved.

NetBSD 3.99.24 (INSTALL_ACPI) #0: Fri Aug  4 14:22:10 CST 2006
         oster@quadrapus:/u1/builds/build69/src/sys/arch/amd64/compile/INSTALL_ACPI
total memory = 8143 MB
avail memory = 7848 MB
timecounter: Timecounters tick every 10.000 msec
timecounter: Timecounter "i8254" frequency 1193182 Hz quality 100
timecounter: selected timecounter "i8254" frequency 1193182 Hz quality 100
mainbus0 (root)
cpu0 at mainbus0: apid 0 (boot processor)
cpu0: AMD Opteron(tm) Processor 246, 2000.06 MHz
cpu0: features: e7dbfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu0: features: e7dbfbff<PGE,MCA,CMOV,PAT,PSE36,MPC,NOX,MMXX,MMX>
cpu0: features: e7dbfbff<FXSR,SSE,SSE2,LONG,3DNOW2,3DNOW>
cpu0: calibrating local timer
cpu0: apic clock running at 200 MHz
cpu0: 16 page colors
cpu1 at mainbus0: apid 1 (application processor)
cpu1: not started
ioapic0 at mainbus0 apid 2 (I/O APIC)
ioapic0: pa 0xfec00000, version 11, 24 pins
ioapic1 at mainbus0 apid 3 (I/O APIC)
ioapic1: pa 0xdfefc000, version 11, 24 pins
acpi0 at mainbus0: Advanced Configuration and Power Interface
acpi0: fixed-feature power button present
timecounter: Timecounter "ACPI-Fast" frequency 3579545 Hz quality 1000
timecounter: selected timecounter "ACPI-Fast" frequency 3579545 Hz quality 1000
ACPI-Fast 24-bit timer
ACPI Object Type 'Processor' (0x0c) at acpi0 not configured
ACPI Object Type 'Processor' (0x0c) at acpi0 not configured
ACPI Object Type 'Processor' (0x0c) at acpi0 not configured
ACPI Object Type 'Processor' (0x0c) at acpi0 not configured
PNP0A03 at acpi0 not configured
PNP0000 at acpi0 not configured
PNP0200 at acpi0 not configured
PNP0100 at acpi0 not configured
PNP0B00 at acpi0 not configured
PNP0800 at acpi0 not configured
PNP0C04 at acpi0 not configured
PNP0501 at acpi0 not configured
PNP0501 at acpi0 not configured
PNP0400 at acpi0 not configured
PNP0C02 at acpi0 not configured
PNP0C02 at acpi0 not configured
PNP0303 at acpi0 not configured
PNP0C02 at acpi0 not configured
PNP0C02 at acpi0 not configured
PNP0C01 at acpi0 not configured
PNP0A03 at acpi0 not configured
PNP0C0C at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
pci0 at mainbus0 bus 0: configuration mode 1
pci0: i/o space, memory space enabled, rd/line, rd/mult, wr/inv ok
vendor 0x10de product 0x005e (miscellaneous memory, revision 0xa3) at pci0 dev 0 
function 0 not configured
pcib0 at pci0 dev 1 function 0
pcib0: vendor 0x10de product 0x0051 (rev. 0xa3)
vendor 0x10de product 0x0052 (SMBus serial bus, revision 0xa2) at pci0 dev 1 
function 1 not configured
ohci0 at pci0 dev 2 function 0: vendor 0x10de product 0x005a (rev. 0xa2)
ohci0: interrupting at ioapic0 pin 20 (irq 9)
ohci0: OHCI version 1.0, legacy support
usb0 at ohci0: USB revision 1.0
uhub0 at usb0
uhub0: vendor 0x10de OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub0: 10 ports with 10 removable, self powered
vendor 0x10de product 0x005b (USB serial bus, interface 0x20, revision 0xa3) at 
pci0 dev 2 function 1 not configured
vendor 0x10de product 0x0059 (audio multimedia, revision 0xa2) at pci0 dev 4 
function 0 not configured
viaide0 at pci0 dev 6 function 0
viaide0: NVIDIA nForce4 IDE Controller (rev. 0xf2)
viaide0: bus-master DMA support present
viaide0: primary channel configured to compatibility mode
viaide0: primary channel interrupting at ioapic0 pin 14 (irq 14)
atabus0 at viaide0 channel 0
viaide0: secondary channel configured to compatibility mode
viaide0: secondary channel interrupting at ioapic0 pin 15 (irq 15)
atabus1 at viaide0 channel 1
viaide1 at pci0 dev 7 function 0
viaide1: NVIDIA nForce4 Serial ATA Controller (rev. 0xf3)
viaide1: bus-master DMA support present
viaide1: primary channel wired to native-PCI mode
viaide1: using ioapic0 pin 21 (irq 9) for native-PCI interrupt
atabus2 at viaide1 channel 0
viaide1: secondary channel wired to native-PCI mode
atabus3 at viaide1 channel 1
viaide2 at pci0 dev 8 function 0
viaide2: NVIDIA nForce4 Serial ATA Controller (rev. 0xf3)
viaide2: bus-master DMA support present
viaide2: primary channel wired to native-PCI mode
viaide2: using ioapic0 pin 22 (irq 5) for native-PCI interrupt
atabus4 at viaide2 channel 0
viaide2: secondary channel wired to native-PCI mode
atabus5 at viaide2 channel 1
ppb0 at pci0 dev 9 function 0: vendor 0x10de product 0x005c (rev. 0xa2)
pci1 at ppb0 bus 3
pci1: i/o space, memory space enabled
nfe0 at pci0 dev 10 function 0: ioapic0 pin 20 (irq 11), address 00:30:48:57:80:b4
makphy0 at nfe0 phy 1: Marvell 88E1111 Gigabit PHY, rev. 2
makphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, 1000baseT, 
1000baseT-FDX, auto
ppb1 at pci0 dev 13 function 0: vendor 0x10de product 0x005d (rev. 0xa3)
pci2 at ppb1 bus 2
pci2: memory space enabled, rd/line, wr/inv ok
ppb2 at pci0 dev 14 function 0: vendor 0x10de product 0x005d (rev. 0xa3)
pci3 at ppb2 bus 1
pci3: i/o space, memory space enabled, rd/line, wr/inv ok
vga0 at pci3 dev 0 function 0: vendor 0x10de product 0x0161 (rev. 0xa1)
vga0: WARNING: ignoring 64-bit BAR @ 0x14
vga0: WARNING: ignoring 64-bit BAR @ 0x1c
wsdisplay0 at vga0 kbdmux 1
wsmux1: connecting to wsdisplay0
pchb0 at pci0 dev 24 function 0
pchb0: vendor 0x1022 product 0x1100 (rev. 0x00)
pchb1 at pci0 dev 24 function 1
pchb1: vendor 0x1022 product 0x1101 (rev. 0x00)
pchb2 at pci0 dev 24 function 2
pchb2: vendor 0x1022 product 0x1102 (rev. 0x00)
pchb3 at pci0 dev 24 function 3
pchb3: vendor 0x1022 product 0x1103 (rev. 0x00)
pchb4 at pci0 dev 25 function 0
pchb4: vendor 0x1022 product 0x1100 (rev. 0x00)
pchb5 at pci0 dev 25 function 1
pchb5: vendor 0x1022 product 0x1101 (rev. 0x00)
pchb6 at pci0 dev 25 function 2
pchb6: vendor 0x1022 product 0x1102 (rev. 0x00)
pchb7 at pci0 dev 25 function 3
pchb7: vendor 0x1022 product 0x1103 (rev. 0x00)
isa0 at pcib0
com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
com0: console
com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo
pckbc0 at isa0 port 0x60-0x64
pckbd0 at pckbc0 (kbd slot)
pckbc0: using irq 1 for kbd slot
wskbd0 at pckbd0 mux 1
wskbd0: connecting to wsdisplay0
attimer0 at isa0 port 0x40-0x43: AT Timer
pcppi0 at isa0 port 0x61
sysbeep0 at pcppi0
pcppi0: attached to attimer0
ioapic0: enabling
ioapic1: enabling
timecounter: Timecounter "TSC" frequency 2000066250 Hz quality 800
timecounter: Timecounter "clockinterrupt" frequency 100 Hz quality 0
md0: internal 5000 KB image area
atapibus0 at atabus0: 2 targets
cd0 at atapibus0 drive 0: <LITE-ON DVDRW SHW-160P6S, , PS0A> cdrom removable
cd0: 32-bit data port
cd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 4 (Ultra/66)
cd0(viaide0:0:0): using PIO mode 4, Ultra-DMA mode 4 (Ultra/66) (using DMA)
wd0 at atabus2 drive 0: <WDC WD800JD-23LSA0>
wd0: drive supports 16-sector PIO transfers, LBA48 addressing
wd0: 76324 MB, 155072 cyl, 16 head, 63 sec, 512 bytes/sect x 156312576 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd0(viaide1:0:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)
wd1 at atabus3 drive 0: <WDC WD4000YR-01PLB0>
wd1: drive supports 16-sector PIO transfers, LBA48 addressing
wd1: 372 GB, 775221 cyl, 16 head, 63 sec, 512 bytes/sect x 781422768 sectors
wd1: 32-bit data port
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd1(viaide1:1:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)
wd2 at atabus4 drive 0: <WDC WD4000YR-01PLB0>
wd2: drive supports 16-sector PIO transfers, LBA48 addressing
wd2: 372 GB, 775221 cyl, 16 head, 63 sec, 512 bytes/sect x 781422768 sectors
wd2: 32-bit data port
wd2: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd2(viaide2:0:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)
wd3 at atabus5 drive 0: <WDC WD4000YR-01PLB0>
wd3: drive supports 16-sector PIO transfers, LBA48 addressing
wd3: 372 GB, 775221 cyl, 16 head, 63 sec, 512 bytes/sect x 781422768 sectors
wd3: 32-bit data port
wd3: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd3(viaide2:1:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)
boot device: <unknown>
root on md0a dumps on md0b
root file system type: ffs
warning: no /dev/console
init: Creating mfs /dev (413 blocks, 1024 inodes)
erase ^?, werase ^W, kill ^U, intr ^C

WTF?

The 270-based system came with fedora installed and boots from disk just fine, 
so it's not "dead"... I notice that it spits out

mainbus0: Intel MP Specification (Version 1.4) (TEMPLATE ETEMPLATE   )

early on, but the 246-based system does not; similarly a number of "mpbios" 
messages are in the 270 dmesg but not the 246...