Yes, there have been problems with authentication/security. There was a
NOTAM (notice to airman) out a couple years ago in teh San Fransisco
area concerning some teenagers that were responding to radio calls to
airliners. Usually its a bit obvious when transmissions come from the
ground because only those within a *very* short distance can here them
unless they are using a pretty serious antenna. Also, these teenagers
were using phrases that were not standard. Aircraft radios work well
because the aircraft are up in the air.

-robert, CFI