Well, since 1.4.1 is out, I tried using the boot floppy off of it.
This is most definitely *not* the same image as the boot floppy from
1.4:

parsley:~/netbsd 513$ dd if=boot.fs-1.4.1 of=/dev/fd0
2871+1 records in
2871+1 records out
parsley:~/netbsd 514$ dd if=boot.fs-1.4 of=/dev/fd0
2880+0 records in
2880+0 records out
parsley:~/netbsd 515$ 

And, what's more, this one gives me an impassible CLAIM failed from
the physical console, whereas the boot floppy with 1.4 worked
flawlessy. Again, I'm inclined to think that my networking problem 
is with something in the kernel on that 1.4 boot floppy, especially as
something else about the kernel must have changed (OF would boot it
from the console before, won't now).

To clarify, the boot floppy image from 1.4 *does* boot on my 7500
150Mhz/604, doesn't send any CLAIM complaints, even when I'm booting
with output-device set to /chaos/control and input-device set to kbd.
It is not, however, capable of properly initializing the the ethernet
device, mc0. This image can be found at:

ftp.netbsd.org/pub/NetBSD/NetBSD-1.4/macppc/installation/floppy/boot.fs

The boot floppy image 1.4.1 does NOT boot my 7500 set to use the
console. I have not tried it from the serial terminal yet, so I don't
know whether it can initialize mc0 correctly or not. This image can
be found at:

ftp.netbsd.org/pub/NetBSD/NetBSD-1.4.1/macppc/installation/floppy/boot.fs

(I did the paths from memory, but I'm pretty sure of them - I've been
typing them into NcFTP by memory for two days now.)

As implied before, I've put the boot image from 1.4 back on my floppy
and I'm proceeding with (privately given) advice from Dave Inskeep
<dinskeep@yahoo.com>, who suggested that I download all the packages
from ftp.netbsd.org, burn them to a CD-R, mount that manually from the
boot floppy, install from a local disk, and specify the CD I just
mounted.

I'll have news on how that went either tonight or tomorrow morning. I
kind of need to eat dinner soon, but I'll probably come back here
after that.

If the CD doesn't work, I'll get around to seeing if the 1.4.1 boot
floppy will boot from the serial terminal (I may anyway, just to find
out).

Also, if anyone's really interested in what this 1.4 floppy is doing
while it's running, I *am* capable of putting truss etcetera on an NFS
mount and using them to see what ping's trying to do when it's very
actively sending 0 packets.

Dan, Bill, do you guys want any more specific information about what's
going on here?

       ~ g r @ granularity.net