Ben,

1) All of X11R6 except the server should compile fairly cleanly.
   I beleive there's already a NetBSD  template (or cf/ file or
   whatever it's called)  for xmkmf.

2) X11R6 uses the DEC "ws" Xserver-kernel interface that was
   introuduced in Ultrix 4.2 or 4.2a.   That interface isn't 
  supported in NetBSD.  One needs to rip out the DEC framebuffer
  ddx code from the X11R5 Xserver and glue it into the X11R6
  Xserver.


3) Failing  that, compile up an X11R5 server and provide the R5
   misc fonts and Most Things Should Work, but the Motif support
   and  R6 exensions  and similar Good Stuff will be absent.


4) Maunel Bouyer is also interested in  putting together a shared-lib
  userland using the OpenBSD toolchain (or GNU, if the relevant
  binutils snapshot is released by then or otherwise available.)
  Maybe you should get in touch with Manuel (bouyer@ensta.fr)

Personally, I seem to be using X11R6pl10 or thereabouts compiled for
Ultrix, using Ultrix compatibility mode, and everything but the
Xserver works Just Fine. (well, I lie to XDM about various things in
its config file, to get Arne's Xserver to work, but that's about all.)

--Jonathan