As a Pakistani-American, I have more than a passing interest in this subject, and I do think that the movie scenario ("the siege") is overblown. I have no doubt that if there is a mass-casualty terrorist attack there will be a backlash and so on, but I dont think the country will slip into martial law or any other nonsense like that.
I think the concept "Mumbai style attack" needs to be defined better for this discussion. The Mumbai attack was NOT some small group of demented Indian Muslims who decided it was their duty to avenge XYZ offenses by going postal on a massive scale. It was a well planned terrorist operation, meticulously put together by a professional terrorist organization (possibly with the support of a certain state agency). The equivalent of that in the US would imply that there is an identifiable foreign-based terrorist group carrying out the attack. In such a situation, how long would it take for anger to be focused on said foreign organization and its sponsors/hosts/friends and away from some sort of mindless arrival of jackboots in the White house?
Personally, I dont see such an attack as very likely because I think organizations capable of such planning and execution are known entities and calculate their moves carefully and will not chose to do something like this at this time.

A "spontaneous jihadi operation" in which some morons in Virginia get together, decide they need to avenge the blood of their brothers in Marja or whatever and go to I-95 and start killing people is a very different matter. Its not out of the question (my guess is that a small but non-zero number of such morons does exist) but its even less likely than Lashkar E Tayyaba deciding to raise their jihad to thermonuclear levels. And there is a very good chance that said morons will give themselves away (possibly to FBI flypaper operations) before they ever buy their guns and drive up to the interstate. And its likely that if they DO get to the interstate, they will not do as much damage as Schmedlap's scary scenario because they are morons, not trained terrorists.
But if it does happen, what would the response be?