Summary followup of, to date, all on/off list responses...
Looking at cvsweb I see:

The netbsd-3 GENERIC kernel is configured for CONADDR=0x2f8,CONSPEED=57600
While netbsd-3 INSTALL kernel is configured for CONADDR=0x3f8,CONSPEED=9600

Presumably those are defaults -- the option lines are commented
out in the conf, cf:
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/arch/i386/conf/GENERIC?f=u&only_with_tag=netbsd-3&logsort=date
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/arch/i386/conf/INSTALL?f=u&only_with_tag=netbsd-3&logsort=date

for the hardware in question, bios serial defaults are
COM1 (3f8h,irq4)
COM2 (2f8h,irq3)  
and serial port speed setting 57600

Exactly one serial port is physically available and review of
netbsd-3-0-RELEASE i386 INSTALL dmesg (photograph of 'more
/kern/msgbuf' within the 3.0 usb cdrom installer) indicates
the following is detected (I don't know what ns16550a means)
http://galis.org//blade-penguincomputing/notes.html

com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo

Clearly there is an off by one port counting in kernel space
vs bios, just to confuse matters -- so I'll refer to the
address whenever possible, also 'COM' for bios and 'com' for
kernel numbering.

The default physical port available is 2f8h (bios COM2), that
address can be changed.

On prior attempts I configured bios console on COM2 with a speed
of 115200, noting that a minicom speed mismatch yields zero serial
data, here's a table of prior attempts:

bios redirection to COM1(0x3f8), zero bios serial
bios redirection to COM2(0x2f8), consdev com0, zero serial
bios redirection to COM2(0x2f8), consdev com1, zero serial
bios redirection to COM2(0x2f8), consdev com2, serial till checksum

the partial success with "consdev com2" indicates bios serial
port numbering at the boot prompt.

On Fri, Aug 25, 2006 at 04:18:38PM -0400, George Georgalis wrote:
> On another USB only amd64 with an otherwise working install,
> com1 is the available serial port, and it does the same thing...
> boots to VGA after checksum to serial.

On that Sun X2100, that's "bios COM1", an amd64 3.1_RC1
NetBSD 3.1_RC1 (GENERIC) #0: Sun Aug 20 22:47:46 UTC 2006
install detect only:

com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo


A minicom speed mismatch could explain serial fail after checksum.
Is it possible for the kernel to pickup the serial port speed as a
hand-off from bios?

Now, I've set bios/minicom speed to the apparent INSTALLER
default, 9600, that yields complete garbage in serial console
through bios and into boot prompt, below is the end of the
session, I'm entering "consdev 2" then "boot"

> ÄÅ_+ 2                                                                        
Å+#+ ÄÅ_Ä+ +.                                                                   
> ÄÄ+                                                                           
ÄÄ+ű °0#:Å+_                                                                   
3437976+5223824+217008=0|97880

serial cleans up for checksum then stops like before.
What else can I try?

// George
 

-- 
George Georgalis, Information Systems Scientist, Metrum Research Group
2 Tunxis Road, Suite 112, Tariffville, CT 06081; direct: 860.670.4346