interesting post. imagination + subversion = new terror idea.
1) make a simple bomb on board;
nitric acid in one bag, with one person. glycerine with another person, another bag.
mix onboard a long flight. (creating nitro glycerine)
wait for turbulence / drop mixing container. tricky i agree, but not impossible.
2) get a job in the airlines on board planes (is this naive?). one day abuse this trust.
3) from a Tom Clancy book: lazers to blind the pilots as they land so they crash instead.
4) an EMP device of some kind (again, is this naive? is it possilbe to do this?) i know you can cause computers to crash by blasing them with powerful microwaves etc. do this to break the fly by wire-ess, with your powerful handheld portable microwave emitter.
5) ...i have once seen a sign in a plane that said "no self inflating rafts in hand luggage"...
surgical implants is the most likley one to succeed so far i'd say.