It's already happened, multiple times. Shoe bomber. Underwear bomber.
Yep, they'll try. Some people will try anything. But the game's different now. You had a virtual guarantee of success up until 9/11 plane #3. After that, the slightest error and you're more likely to go down in history as the loser guy who got stomped.
Yep, organisations like HAMAS who used to take over planes to exert political pressure must be pissed at Al Queada. Before they used to be able to take over a plane and have everyone placidly sit there whilst they get one of their buddies freed from a western jail. Now they're just going to get ganked.
I think it was Bruce Schneier who said the only two security improvements that came out of 11 September are that the cockpit door is locked and passengers will fight back.