Strange - I have almost the same machine, also use serial terminal, and it
works very well.
As you can see, I'm using 1.6_STABLE from November.
Have you tried 1.6_STABLE or only 1.6 as released?

My dmesg:

[ using 558472 bytes of netbsd ELF symbol table ]
consinit: using prom console
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002
    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 1.6_STABLE (GENERIC) #0: Fri Nov 22 12:41:51 MET 2002
    bouyer@java:/home/tmp/alpha/obj/sys/arch/alpha/compile/GENERIC
DEC 3000 - M600, 175MHz, s/n 
8192 byte page size, 1 processor.
total memory = 192 MB
(2048 KB reserved for PROM, 190 MB used by NetBSD)
avail memory = 168 MB
using 1228 buffers containing 9824 KB of memory
mainbus0 (root)
cpu0 at mainbus0: ID 0 (primary), 21064-1
cpu0: Architecture extensions: 2<FIX>
tcasic0 at mainbus0
tc0 at tcasic0: 25 MHz clock
ioasic0 at tc0 slot 7 offset 0x0: fast mode
le0 at ioasic0 offset 0xc0000: address 08:00:2b:be:e4:bf
le0: 32 receive buffers, 8 transmit buffers
scc0 at ioasic0 offset 0x100000
scc1 at ioasic0 offset 0x180000: console
mcclock0 at ioasic0 offset 0x200000: mc146818 or compatible
bba0 at ioasic0 offset 0x240000
audio0 at bba0: full duplex, mmap
tcds0 at tc0 slot 6 offset 0x0: TurboChannel Dual SCSI (baseboard)
tcds0: fast mode set for chip 0
asc0 at tcds0 chip 0: NCR53C94, 40MHz, SCSI ID 7
scsibus0 at asc0: 8 targets, 8 luns per target
tcds0: fast mode set for chip 1
asc1 at tcds0 chip 1: NCR53C94, 40MHz, SCSI ID 7
scsibus1 at asc1: 8 targets, 8 luns per target
scsibus0: waiting 2 seconds for devices to settle...
sd0 at scsibus0 target 1 lun 0: <Quantum, VP32210, 81HA> SCSI2 0/direct fixed
sd0: 2103 MB, 4243 cyl, 8 head, 126 sec, 512 bytes/sect x 4308352 sectors
sd0: sync (100.0ns offset 15), 8-bit (10.000MB/s) transfers, tagged queueing
scsibus1: waiting 2 seconds for devices to settle...
sd1 at scsibus1 target 2 lun 0: <CDC, 94181-15, 0297> SCSI1 0/direct fixed
sd1: 315 MB, 791 cyl, 15 head, 54 sec, 512 bytes/sect x 645300 sectors
sd1: async, 8-bit transfers
sd2 at scsibus1 target 4 lun 0: <CDC, 94181-15, 0297> SCSI1 0/direct fixed
sd2: 315 MB, 791 cyl, 15 head, 54 sec, 512 bytes/sect x 645300 sectors
sd2: async, 8-bit transfers
Kernelized RAIDframe activated
root on sd0a dumps on sd0b
root file system type: ffs