Security Concern: Unsigned Windows Executable

[reynt0]

On Fri, 5 Jun 2009, Robert J. Hansen wrote:
  . . .
> I do not believe it is possible to ensure the security of your computers
> or your communications when in the presence of an active MitM done by a
> competent attacker.
  . . .

I'm curious.  Not counting DOS (which can always be done
by cuting your outside wires,if nothing else), isn't there
*any* way to get some things done despite competent MitM?
Like exchange public keys in person then go home and use
those to communicate?  (Note, this is just MitM in question,
not attack on your host, etc.)