Well, I have purchased my Cabletron, and a 10BaseT tranceiver for my sun3
and it works! It took a bit of hacking of the soldering kind to make an
adaptor to that funny mac 25 pin scsi connector. Also, I had a bit of 
puzzling to get the 10BaseT crossover cable made properly (I wasn't sure
how the pins on a RJ45 connector are numbered).

When I followed the instructions and tried to ping the sun3 I got
"no route to host" message. I couldn't get it to work until I did something
like "route add -net 10.0.0.0 10.0.0.1 -interface". I don't understand
the routing commands very well. Should I expect to have to do that?
One thing, I had a ppp running which set up a default route with the
other end of the ppp link as gateway. Maybe that causes the problem?

Regarding moving the etherd functionality into the kernel: I haven't
looked at the scsi code, but surely scsi commands are executed from
inside the kernel for multiple reasons. Inquiry, Request Sense, and
write file mark commands must be initiated inside the kernel and I
assume block IO and paging/swapping is not done in a process context.

Ian