My understanding of the issue:

When OpenBSD split from NetBSD in about 1997 or 1998, the OpenBSD people did
not, for whatever reason, choose a distinguishing way to mark their executables.
They continued to produce "NetBSD" executables.

Over time, NetBSD and OpenBSD both evolved the kernel, to the point where the
binaries were not compatible.

Since that time, NetBSD has converted the remaining architectures to ELF style
executables.  More resently, I am told that OpenBSD did likewise.  (Some NetBSD
systems have *always* been ELF, I'm told, but (for example) i386 used a.out
style executables for a long time.)

Under ELF, I gather that OpenBSD at last has its own distinct mark for binary
files, so it is possible to run either type on a single system.  At least in
principle.  I haven't heard of anyone working on this, but it might be
under way.

NetBSD has been able to run GNU/LINUX, FreeBSD, BSD/OS, and numerous other
types of programs for years.


Anyway, that's my understanding of the situation.  Take it with a grain of
salt, since I'm just repeating what I've picked up via osmosis.


-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/