The question you are asking, would be better asked:
Can, what some label 'terrorism', be considered an acceptable form of Jihad and/or warfare against oppression?
I don't know the answer. Here are a few points that I do know:
1) Subjecting innocent women and children to ANY type of violence is NOT permitted in Islam. I believe, when Hazrat Umar (the second righteous Caliph), took Jerusalem, he asked his soldiers not to a) harm children, b) harm women, and c) cut down any trees. Similar campains of war, fought by Muslims against aggressors, have insured that women and children are safe-guarded from harm.
2) Further, war, as I understand it is not to be waged in the sacred month of Ramadan - which is NOW.
3) Muslims are commanded to wage war against those who wage war against them: "And wage war on all of the idolaters as they are waging war on all of you. And know that Allah is with those who keep their duty (unto Him). (9:36)"
4) Once aggression is rooted out, Muslims must desist from waging war. "So, if they hold aloof from you and wage not war against you and offer you peace, Allah alloweth you no way against them. (4:90)"
So if the actions of these individuals (hijackers, terrorists, freedom fighters, mujahideen) or whatever you want to call them, fit into the category of a just struggle, they're actions and intentions would be in the right if - a) they did not wage this war in Ramadan, b) they did not harm anyone who is not an aggressor, c) they treated prisoners (which is what the hostages properly, d) they freed women and children right away, e) they freed those who are not in any way responsible for the conditions of aggression against Muslims (ie, non-Indians on board the plane).
I don't think there actions are justified Islamically, I think most would agree. It was the wrong time, and the wrong venue. They would be better off continuing to fight a ground battle with the oppressive Indian military, and not the general population of India.
Achtung