Why not simply have totally independent networks?
A good idea for many reasons already discussed. I would even be tempted to make the aircraft's networks to a different standard from the norm, at least with regard physical connectors. An ethernet RJ-45 could let you plug (and leave) all manner of undesirable devices connected. I don't know enough about the wireless/datalink stuff to comment.
This is one place for (quality) comment and analysis in the IT world on this and other risk s:
http://catless.ncl.ac.uk/Risks/25.1.html#subj2