I'm interested in advice about how to build a native ELF userland, and
   most importantly, toolchain for NetBSD SPARC.  Is there a relatively
   painless way to do this?

i will have a look at this with the NetBSD gnu toolchain sources
that the alpha and other ports use.
   
   Has anyone considered what's involved in emulating a NetBSD environment on
   NetBSD.  Or rather, a different NetBSD environment on NetBSD.  I want to
   switch to an SPARC elf64 format but still be able to run SPARC a.out
   executables.

actually, compat_m68k4k is compatibility with m68k 4K pagesize executables,
that i think at least the hp300 port uses for backwards compat.  i'm sure
it wouldn't be too hard.
   
   On a similar note, what is the significance of ARCH and MACHINE_ARCH?   I
   remenber that the m68k ports has ARCH set to m68k and MACHINE_ARCH to the
   particular machine type: sun3x, mac68k, amiga.  Most of the compilation is
   based on ARCH; all the machine dependent arch directories seem to use
   ARCH.  I want to use a different set of arch directories but maintain
   compatibility.  
   
   What I really want is some way to use a different arch, say sparc64, if it
   exists, but default to the standard sparc arch if it doesen't. Is that
   possible?  What would start to break if I switched to a sparc64 ARCH?  

gordon and jeremy may have ideas for this -- look at what the sun3x port
has had to do to achieve this.
   
   P.S.  Anyone consider switching native NetBSD SPARC to elf executables?


i think this is the eventual goal.