I'm having a bit of trouble with the ps/2 busmouse driver in
   NetBSD-1.0...

The PS/2 mouse driver in 1.0 is a bit broken.  The version in -current
should work better.

   [...]; the server starts up, but the first xterm it tries to start
   dies with "xterm: no ptys available", [...]

Are you sure you have device nodes for the ptys?

cd /dev
sh MAKEDEV pty0 pty1

That should device nodes for 32 ptys.

   Finally, has anyone worked on pulling the driver for Dec 21040-based
   PCI ethernet cards from FreeBSD over to NetBSD?

I've looked at it.  Its configuration strategy needs work, and I
haven't had time to work on it.  I have a version that's mostly
converted which I can send you.